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Geographic Information Systems

Mapping Raster Data

Previous: Mapping Coordinate Data

Following: Editing Map Data

Geographic data is commonly in the form of rasters, such as scanned maps, aerial or satellite photos, and elevation grids.



  1. Determining the Characteristics of a Raster Image

  2. Choosing the Best Visualization of a Raster

  3. Downloading Elevation Data from the National Map

  4. Setting Up a Default Geodatabase

  5. Clipping an Elevation Raster to a Smaller Extent

  6. Determining Watersheds with an Elevation Raster

  7. Georeferencing a Scanned Map

Getting Started

Since this tutorial will be using specific maps and data, the first step is to make your own copy of the tutorial data.

Set Up 1: Getting the Tutorial Data

  1. In the Windows Explorer, navigate to the network drive  K: (aka \\Software\Winsoft), open the folder  Maps, and then open the folder  Introduction to GIS.
  2. Drag the folder  MappingRasters and its contents to either:
    1. your network drive  U:, e.g. into the folder  My Documents; or
    2. the local hard drive  C:, e.g. onto your Desktop.

The folder  MappingRasters contains the following layers:


Since some uses of ArcGIS require names without spaces or special symbols, do not rename the folders or files.

Set Up 2: Initializing ArcMap and Adding Data

  1. Start up the  ArcMap software (see Constructing and Sharing Maps for details).
  2. Click on the button Add Data Icon Add Data.
  3. In the dialog Add Data, navigate into the folder  MappingRasters; if necessary, make a new connection to it (see Constructing and Sharing Maps for details).
  4. In the folder  MappingRasters, click on the file  amherst_2004.sid and then ctrl-click on the files  amherst_elevation.tif,  q117894.tif, and amherst_boundary.lyrto add them to the selection. The first is an orthophoto of Amherst, the third is an elevation model, and the third is a topographic quadrangle, a scanned map prepared by the United States Geological Survey.
  5. Click on the button Add.

Exercise: Zoom into the lower left corner of this map and click on the quandrangle on and off; how well do the rasters line up with the boundary layer?

The Structure of Raster Data

Geographic raster images have some basic features but still come in a wide variety of formats, which are used for specific purposes.

The Spatial Location of Raster Data

Recall that raster data, such as orthophotos or scanned maps or elevation models, consist of a grid of pixels whose values say something about the surface of the Earth:

Like vector data, the raster data used by GIS will always be defined in one particular spatial reference, where it is a rectangular grid.

However, raster data must also provide the following information relative to the coordinate system of the spatial reference:

  • The location of one pixel (e.g. the center of the upper-left pixel);
  • The size of its pixels, e.g. meters or degrees, which will be either square (usually) or rectangular (rarely);
  • The amount of rotation of the raster relative to the easting and northing directions.

This transformation information allows the position of every pixel to be calculated and correctly displayed relative to other data, and such rasters are said to be georeferenced.

Not surprisingly, when a raster is reprojected to another spatial reference, it will appear with a distorted shape:

Massachusetts State Plane

The transformation information is stored in a number of ways, such as a separate world file, commonly provided on the Internet for georeferenced rasters, e.g. .tfw.

Unfortunately the world file format does not also include the spatial reference, so you must look for that information separately, as an associated .prj file or as a textual description that you must incorporate in the same way as for vector data.

Both types of information will be stored in a .aux file, if that’s available.

The Representation of Pixel Data

Computer and digital television screens are based on a physical grid of picture elements or pixels, each of which is a triplet of the primary colors red (R), green (G), and blue (B).

The pixels are close enough together that they are visually merged, and the mixture is perceived by the brain to be one of the many colors that are visible to the human eye.

ArcGIS Color SelectorArcGIS Color PaletteFor example, equal amounts of red and green with no blue produces yellow (see the color selector at the right). Equal amounts of all three are a shade of gray. In particular, when all three are zero, the color is black, and when all three are their maximum value, the color is white.

Current digital technology usually describes each of the three values by an integer between 0 and 255, allowing the display of a total of 2563 = 16,777,216 colors. These provide a good representation of the colors that the eye can see (though this is not the entire gamut of color).

Wherever ArcGIS lets you choose a color, it will provide a palette of common colors, shown at the right, but it also lets you click on the button More Colors… to bring up the dialog Color Selector, letting you individually select the RGB values.

Raster data is stored in a number of different formats that may or may not explicitly include color information. Some of the more common formats are:

  • Impervious SurfacesIndexed Color PaletteColor Map or Indexed Color: Each grid cell has a single value that is an index into a palette of colors stored with the raster:

    Besides limited-color printed materials, such as the scanned topographic map to the right (original source the U.S. Geological Survey), these values may also represent categorical data such as impervious surfaces (far right).

    If a color map is not provided with the raster, ArcGIS lets you assign colors randomly with discrete colors or to your choice of colors with unique values.
  • Grayscale: A single value that is commonly displayed using a mathematically defined ramp ranging between black and white:

    0 Grayscale Ramp 255

    Such a ramp is used for “black-and-white” photographs as well as other data.

    Non-photographic data could also be displayed with a color ramp such as

    0 Elevation Color Ramp 255

    which is commonly used for elevation.
  • RGB: A triplet of values that is displayed on your computer screen as a visually merged color:

    Red: 0 255
    Green: 0 255
    Blue: 0 255

    This format is used for color photographs, along with satellite imagery that may substitute another wavelength of light such as infrared, known as Color-Infrared (CIR).

The number of values assigned to each pixel is referred to as the number of bands or channels. Multispectral satellite imagery can have seven or more bands per pixel, but computer display technology will show at most three of them at once.

The values used for each pixel band may be one of several numeric types:

Raster Pixel Types

Pixel Type Pixel Depth Minimum Value Maximum Value
Unsigned Integer 8 bit = 1 byte
  16 bit = 2 bytes
  32 bit = 4 bytes
Signed Integer 8 bit = 1 byte
  16 bit = 2 bytes
  32 bit = 4 bytes
Floating Point (Real) 32 bit = 4 bytes
-3.4 x 1038
1.2 x 1038
Double Precision (Real) 64 bit = 8 bytes
-2.2 x 10308
1.8 x 10308

Generally speaking, the greater the depth, the larger the file size of the raster, so smaller depths are used when possible.

For example, if you have an elevation range that varies between sea level (0 m) and 200 m, and you don’t need fractional values, you could use one-byte unsigned integers.

If you have a color photograph, the three RGB channels will need at least three integer bytes; but because of the power-of-two design of computer architectures, they are commonly stored as a four-byte quantity.

The fourth byte will sometimes hold information about a pixel’s degree of transparency (or its inverse, opacity); it is then known as an alpha channel.

For most imagery formats ArcGIS can view the individual color channels. When opening such images, ArcMap and ArcCatalog treat them as “folders” that open up to list Band_1, Band_2, …. So if you want to view the combined format, you can’t double-click on the file, you need to click on it once and then click the button Add.

Often a rectangular raster will include pixels that cover locations that can’t be assigned actual values, e.g. in an elevation data set that might lie over water. Such pixels are typically assigned a value or value combination that is understood to represent NoData. If ArcGIS can determine what that “color” is, it will display it as completely transparent (this special value may be stored in an associated .aux file).

The File Formats of Raster Data

Rasters may be stored in a number of different formats, which may or may not be compressed to save space. The greatest compression is usually achieved by using a lossy compression format that will not perfectly reconstruct the original data.

Raster File Formats

File Format File
World File
Pixel Type(s) Compression Description
Windows BitMaP .bmp .bpw, .aux Colormap
None (usually) The standard Windows image format, very basic.
Graphics Interchange Format .gif .gfw, .aux Colormap Lossless A compressed image format that is commonly used on the Internet for images with simple colors and structures, e.g. line drawings and simple scanned maps.
Portable Network Graphics .png .pgw, .aux Colormap
Lossless A compressed image format that is replacing .gif on the Internet due to its better compression and more flexible pixel types.
Tagged Image File Format .tif, .tiff internal
Optional lossless Commonly used for photographic work as well as scientific imaging, its use on the Internet is uneven due to its many variations.

A new version of the format, GeoTIFF, embeds transformation information in the TIFF header.
Joint Photographic Experts Group .jpg, .jpeg .jpw, .aux Grayscale
Lossy (can be lossless) An open standard that is commonly used on the Internet for photographs and other images with many gradations of color.
Joint Photographic Experts Group 2000 .jp2 .j2w, .aux Grayscale
Lossy (can be lossless) A newer open standard that stores multiple resolutions (scales). It is not yet completely supported on the Internet.
Multiresolution Seamless Image Database .sid internal
Lossy (can be lossless) A proprietary format that stores multiple resolutions (scales). Supported on the Internet only via web browser plug-in.
GRID None .aux Grayscale
Lossless ESRI’s proprietary image format, not supported on the Internet.

The .aux format is ArcGIS-specific, so it will be less common even though it’s more convenient, including both transformation and projection information. If present it will take precedence over a world file.

In addition to the auxiliary files, another associated ArcGIS file you may come across is the pyramid file, with file extension .rrd. It holds lower-resolution versions of the original image to facilitate rapid display when it’s viewed at smaller scales. Multi-resolution files such as JPEG2 and MrSID include pyramids as part of their definition. When you add other formats lacking a pyramid file to a map, ArcGIS will ask if you want to build one; generally this is a good idea.

ArcGIS also stores statistics information for images in .xml files.

Rasters are far more prevalent on the Internet than other formats such as shapefiles or even XY tables, because they are often images that can be directly viewed. Make sure to also download associated world files, projection files, et al.!

Layer Properties Dialog, Source PageProcedure 1: Determining the Characteristics of a Raster Image

  1. In ArcMap Icon ArcMap, in the Table of Contents, double-click on the raster of interest, e.g.  amherst_2004.sid ,  amherst_elevation.tif, or  q117894.tif.
  2. In the dialog Layer Properties, click on the tab Source.
  3. Read the table Property | Value:
    1. In the section Raster Information, you should note the following:
      • The number of Columns and Rows in the raster (in this example, they are equal so it’s square)
      • The Cellsize (X, Y) (pixel size) in the units of the coordinate system (in this example, it’s again square).
      • The file Format;
      • The Number of Bands per pixel;
      • The Pixel Type and Pixel Depth;
      • If a Colormap is used;
      • If a NoData value is assigned;
      • If Compression is used.
    2. Scrolling down to the section Spatial Reference, you should note what that is, and also its Linear Unit (if it has one).
    3. Scrolling up to the section Extent, note the distance the raster covers in each direction.
  4. Click on the button OK to dismiss the dialog.

Exercise: How does the other raster differ?

Symbolizing Raster Data

ArcGIS will commonly display rasters in a non-uniform manner, which is often the best way to make their variations more visible, but is not always the best approach.

Visualizing the Data

The different values of the cells in a raster can vary dramatically, often in a way that is hard to visualize on a computer screen. This is because

Value statistics for a raster layer.ArcGIS will usually analyze the statistics of the cell values to determine the best way to display the data. For each band, the statistics include:

  • Minimum value;
  • Maximum value;
  • Mean value, which is an estimate of the middle (symbolized by μ);
  • Standard deviation, which describes how the values are distributed around the mean value (symbolized by σ).

These values are included in the source properties described in Procedure 1, as in the example here for the layer  amherst_elevation.tif, whose units are feet (though this must be determined from the layer’s metadata).

The statistics can be displayed and visualized by looking at the histogram of the values.

Amherst Digital Elevation ModelProcedure 2: Choosing the Best Visualization of a Raster

  1. In ArcMap Icon ArcMap, in the Table of Contents, double-click on the raster of interest, e.g.  amherst_elevation.tif.
  2. In the dialog Layer Properties, click on the tab Symbology.
  3. In the list Show:, if it’s not already selected, click on the symbolization Stretched, which refers to a translation of all possible values in the raster to a particular color ramp, which by default for a single-band raster is grayscale:

    Layer Properties Dialog, Source Page

    The Stretched dialog will display the minimum and maximum values in the raster, which in this case represent feet of elevation.

    To be visible, a set of raster values must be stretched between black and white. ArcGIS provides a number of different methods to stretch values, and by default uses a Type: of Standard  Deviations.
  4. Amherst DEM HistogramClick on the button Histograms, and the distribution of raster values will appear in a new window:
    • All elevation values along the horizontal;
    • The number of pixels with a particular elevation along the vertical.

    As you can probably tell from the raster image above, there are more low-elevation (darker) values than high-elevation (lighter) values in the raster, and this is represented in the histogram, which skews towards the lower values.

    Additional statistics, the mean value μ and standard deviation σ, are also shown.

    By default the minimum and maximum raster values are not simply stretched between the visible values of black (0) and white (255). Instead the stretch type Standard Deviations focuses on the range between the two values μn σ and μ + n σ, where μ is the mean value, σ is the standard deviation around the mean, and n is a user-defined value, by default equal to 2. Any values outside of this range are folded into black and white. This has the purpose of making the region around the mean value more visible, which is most useful when the histogram values are clustered in part of the range.

    In the case of this elevation raster, this means that the values

      μn σ = 480 – 2 * 327 = –176 → minimum value (8)
      μ + n σ = 480 + 2 * 327 = 1,134

    are assigned to black and white, respectively, and every value in between is stretched linearly. Everything above 1,134 is also mapped to white, which slightly enhances the contrast in the lower elevations but washes out the higher elevations.

  5. Click on the button OK to dismiss the Histogram dialog.
  6. Back in the dialog Symbology, in the area Stretch, click on the menu Type:, and select the menu item Maximum-Minimum.

    This option applies the stretch over the full range of the data, which is usually better when the data isn’t clustered.

    You also have the option of editing the maximum and minimum values that are stretched, which is useful when you are focusing on only a portion of an elevation raster and want to bring out its detail, or have a set of rasters with different minima and maxima that you want to symbolize in the same way.
  7. Click on the button OK to save your changes and close the Symbology dialog.

Exercise: How does the other raster differ?

A fuller discussion of data statistics can be found here.

Analyzing Watersheds

Watersheds are defined by relative elevation, and elevation data in raster format is necessary for their analysis.

Amherst Elevation Raster Elevation Contours and Control Points Watersheds are regions on the surface of the Earth where all water flows downhill to a single point on a stream or river, or into a particular body of water. These are known as pour points.

Water is an important resource that carves landscapes, fills reservoirs, and carries soil and pollutants, so determining the extent of watersheds is important for geological, agricultural, social, and political applications.

“Downhill”, of course, means from higher elevation to lower elevation, so elevation data is required to locate watersheds.

Many quantities such as elevation and precipitation vary continuously across the surface of the Earth; they are called fields.

Fields are generally approximated by sampling them at specific locations, called control points (e.g. the red points in the image at the right), and inferring other values in-between using geographic algorithms.

Fields can be represented by a number of formats, such as the common visual representation in the image to the near right of contour lines that have a constant elevation. “Uphill” and “downhill” are always perpendicular to these lines, and the closer together they are, the more rapid the change in elevation.

Elevation rasters such as the one to the far right of the same location are particularly useful for calculations that can be applied uniformly to each grid cell, known as raster arithmetic, and are therefore commonly used in watershed analysis.

Procedure 3: Downloading Elevation Data from the National Map

  1. In your web browser, visit The National Map.
  2. Search for the area you want (it understands names like Bare Mountain, MA);
  3. Zoom to the extent you need;
  4. Click on the button Download&nbps;Data at the top of the page;
  5. In the dialog Download options, you can choose several ways to describe the data you want, e.g. download by current map extent;
  6. In the dialog for USGS Available Data for Download, check on the box  Elevation DEM Products;
  7. Click the button Next;
  8. A list of available rasters will appear; ArcGrid, GeoTIFF, and TIFF will be the easiest to input to ArcGIS.

    The most detailed will be 1/9 arc second (~3 m), but that will also be the largest (perhaps hundreds of MB each).

    Also consider the date of the rasters and if your area of interest requires the most recent or not.

    Click on the name of each raster to see their extent; they often cover only part of the area you’ve selected.

    Check the box next to the rasters you want to download.
  9. Click the button Next;
  10. In the panel Cart, click on the button Checkout.
  11. Provide your e-mail address;
  12. Click on the button Place Order;
  13. Check your e-mail for the download links they will send you.

Sources of elevation data include:

In certain areas the resolution of raster data can be as small as 3 inches or 0.25 ft per pixel.

Be aware that vertical units are typically stored only in human-readable metadata, and may differ from the horizontal units (e.g. feet vertical and meters or degrees horizontal).

The basic steps in watershed analysis are:

  1. Clip the raster to a small area around the watershed to reduce its size;
  2. Fill in any sinks in the elevation raster, which are depressions where water could collect instead of flowing downhill (often these are artifacts of interpolation from the control points or bodies of water that actually do have an egress);
  3. Determine the flow direction at each grid cell, which tells you how water flows downhill;
  4. Determine the flow accumulation at each grid cell, which lets you accurately determine the stream channels within the raster and the location of possible pour points;
  5. Choose one or more pour points;
  6. Determine which grid cells make up the watershed of each pour point.

Watershed analysis is an example of geoprocessing, a set of calculations that transform geographic data so that information can be extracted from it.

Most geoprocessing tools are found in ArcToolbox, and run in the background by default. Until you are comfortable with using these tools, it is highly recommended that you disable background processing (even with Version 10.2 of ArcGIS, background processing sometimes fails when foreground processing succeeds).

Because geoprocessing often involves a series of steps, where the the output of each step is the input to the next one, ArcGIS defines a default workspace that automatically appears as the output location in  ArcToolbox tools. However, it’s generally better to define a new one for every project.

Procedure 4: Setting Up a Default Geodatabase

Geodatabases are collections of related content in multiple formats, specifically vector feature classes, raster datasets, and tables.

Geodatabases are the native data format for the most recent versions of ArcGIS, supplanting the older vector shapefile and raster GRID formats that are still commonly used for data interchange.

Geodatabases can be stored in database systems such as Access, Oracle, and PostgreSQL, or in a special kind of folder called a file geodatabase, which is most convenient for individual work.

ArcGIS defines a default file geodatabase, C:\Users\username\Documents\ArcGIS\Default.gdb, which is used as the default output workspace for  ArcToolbox tools.

Generally, though, it’s better to create a new default file geodatabase for each project, usually in your map’s Home folder or some other accessible location.

  1. To work with files and workspaces within ArcMap Icon ArcMap, a number of tools are available through the Catalog:
    • CatalogIn ArcMap Icon ArcMap, look for the vertical tab Catalog along the right edge of the ArcMap window and point at it, whence it should automatically pop out;
    • If the vertical tab isn’t present, look in the toolbar Standard and click on the button Catalog.

      The Catalog will appear in its own window but can be “pinned” to the right edge of the ArcMap window, which will keep it out of the way until you need it:
      1. Begin to drag the window and a set of “pinner” buttons will appear; move the cursor on top of the Winder Right Edge Pinner right-edge pinner and release.
      2. Click the button Auto Hide at the top of the window, so that it will go away automatically when you aren’t pointing at it.
  2. If the  Home folder isn’t visible at the top of the catalog, save your current map document in a good location, preferably in the same folder as the data it uses, e.g.  MappingRasters.
  3. Create a new file geodatabase by right-clicking on your home folder to bring up a contextual menu, then pointing at the submenu New , and finally selecting  File Geodatabase.
  4. Give the file geodatabase an appropriate name, e.g. hadleyreservoir.gdb.
  5. Right-click on the new file geodatabase to bring up a contextual menu and select  Make Default Geodatabase.

Once you have your default geodatabase defined, the next step in watershed analysis is to clip your elevation raster to the area you want to study, which will speed up the process since elevation rasters are often quite large.

Procedure 5: Clipping an Elevation Raster to a Smaller Extent

For this procedure you must already have an elevation raster available, e.g. the raster  amherst_elevation.tif.

  1. You can clip the elevation raster using another raster or a polygon as a mask; only the parts of the raster that fall inside of the mask will be extracted (note: this is the inverse of how this term is usually used, where what is masked is removed).

    To create a polygon mask:
    1. If necessary, add the toolbar Draw, by menuing Customize > Toolbars > Draw:

      Toolbar Draw

      Dock it with the other toolbars if you want.
    2. Polygon MaskIn the toolbar Draw, click on the tool  Rectangle; if you see another shape tool in the location shown above, click on the  menu next to it and choose  Rectangle.
    3. In the toolbar Draw, change the  Fill Color to No Color and the  Line Color to a bright contrasting color.
    4. Zoom to the area of interest, analyze the terrain to roughly estimate the watershed’s extent, and then click-and-drag out a rectangle that covers that extent — it is best to somewhat overestimate its size.

      In the graphic on the right, the contours on the topographic map help determine the approximate area over which water flows into the enclosed reservoir.
    5. Once you complete your drag-out of the rectangle, it will be selected and display handles (aqua boxes) around its edge; you can then adjust its position:
      • You can click-and-drag the handles to reshape the rectangle;
      • With the tool  Select Elements you can click on the outline of the rectangle (away from the handles) and drag it around; you can also move it using the arrow keys;
      • If you deselect the rectangle, you can select it again with the tool  Select Elements (both in the toolbar Draw and the toolbar Tools);
      • If you want to redraw the rectangle from scratch, make sure it’s selected and press the key delete to remove your earlier attempt.
    6. To create a mask from a selected rectangle, in the toolbar Draw, in the menu Drawing, choose Convert Graphics to Features….
    7. In the dialog Convert Graphics to Features, the Output shapefile or feature class: is best located with your other data, e.g. in the file geodatabase hadleyreservoir.gdb, and with a name such as area.
    8. Click on the checkbox  Automatically delete Graphics after conversion.
    9. When asked Do you want to add the exported data to the map as a layer?, click on the button Yes.

      The graphic will be converted to a feature class, e.g.  area, and the former will be deleted from the map and the latter added to the map.
  2. In ArcMap Icon ArcMap, turn on the Spatial Analyst extension, which provides a specialized set of tools for working with rasters:
    1. Menu Customize, then click on the menu item Extensions….
    2. In the dialog Extensions, click on the checkbox  Spatial Analyst.
    3. Click on the button Close.
  3. Polygon-masked RasterTo mask the elevation raster with a polygon or another raster:
    1. In the window  ArcToolbox, double-click on  Spatial Analyst Tools, then on  Extraction, and finally on Extract by Mask.
    2. Drag the elevation raster, e.g.  amherst_elevation.tif, from the Table of Contents into the dialog  Extract by Mask and the field Input raster.
    3. Drag the mask, e.g. area, from the Table of Contents into the dialog  Extract by Mask and the field Input raster or feature mask data.
    4. Choose an appropriate location for the Output raster, e.g. in the file geodatabase hadleyreservoir.gdb, and give it a name such as elevation.
    5. Click on the button OK.
  4. When the tool has completed successfully, click on the button Close.

Once you have a raster covering the extent of the watershed, you can determine more precisely its area by analyzing the flow of water over the terrain.

Procedure 6: Determining Watersheds with an Elevation Raster

For this procedure you must already have an elevation raster available, e.g. the raster  amherst_elevation.tif or its clipped version,  hadleyreservoir.gdb/elevation, which covers the smaller area shown above and will therefore process faster.

  1. In ArcMap Icon ArcMap, turn on the Spatial Analyst extension, which provides a specialized set of tools for working with rasters:
    1. Menu Customize, then click on the menu item Extensions….
    2. In the dialog Extensions, click on the checkbox  Spatial Analyst.
    3. Click on the button Close.
  2. Most rasters will have some grid cells with lower elevation than any of their neighbors, which are called sinks. These are likely to be artifacts, e.g. ponds whose egress is not visible at the raster resolution.

    We will therefore begin by filling in any sinks in the elevation raster:
    1. Double-click on  Spatial Analyst Tools, then on  Hydrology, and finally on Fill.
    2. The dialog  Fill has a number of text fields and buttons in it.

      If visible, click on the button Show Help >>; it will display brief bits of information about the tool, starting with an overview and providing details about each available control (field, button, menu) as you select them.

      More comprehensive information about the tool can be obtained by clicking on the button Tool Help.
    3. Drag the elevation raster, e.g.  elevation, from the Table of Contents into the field Input  surface raster.
    4. Choose an appropriate location for the Output surface raster, e.g. in the file geodatabase hadleyreservoir.gdb with a name such as fill.
    5. Occasionally sinks are actually sinkholes that provide significant recharge for groundwater, and should not be filled; setting the field Z limit to an appropriate value can preserve these features.
    6. Fill Tool ExecutingClick on the button OK.

      The dialog will be replaced with another that shows how the process is Executing. When it is Completed, the output surface raster will be added to the Table of Contents. Notice that the lowest elevation is now slightly higher.
    7. Click on the button Close.
  3. To determine how water flows from grid cell to grid cell as it descends, calculate the flow direction:
    1. In the window  ArcToolbox, double-click on Flow Direction.
    2. Drag the filled elevation raster, e.g.  fill, from the Table of Contents into the dialog  Flow Direction and the field Input surface raster.
    3. Flow DirectionChoose an appropriate location for the Output flow direction raster, e.g. in the file geodatabase hadleyreservoir.gdb with a name such as flowdirection.
    4. Click on the button OK.
    5. The dialog will be replaced by another displaying how the process is proceeding; when it is Completed, click the button Close.

      The output flow direction raster will be added to the Table of Contents. The values in the raster correspond to the flow direction as follows:
    6. 64 — N
      32 — NW 128 — NE
      16 — W 1 — E
      8 — SW 2 — SE
      4 — S

      Note that these directions are based on the projected coordinate system (northing and easting), not necessarily the true cardinal directions.

      By default the different colors assigned to these values are chosen randomly, but if you symbolize the raster with a color-wheel ramp Color Wheel Ramp the visual relationship of the directions may be more understandable. You can also relabel the symbols as shown above.

  4. To determine which grid cells collect the most water, in particular the stream channels within the raster, calculate the flow accumulation:
    1. In the window  ArcToolbox, double-click on Flow Accumulation.
    2. Drag the flow direction raster, e.g.  flowdirection, from the Table of Contents into the dialog  Flow Accumulation and the field Input flow direction raster.
    3. Choose an appropriate location for the Output accumulation raster, e.g. in the file geodatabase hadleyreservoir.gdb with a name such as flowaccumulation.
    4. Click on the button OK.
    5. The dialog will be replaced by another displaying how the process is proceeding; when it is completed, click the button OK.

      The output flow direction raster will be added to the Table of Contents. Larger values (in white by default) are downhill, with more grid cells above them from which water could flow into them.

      Note that the stream channels this tool finds may not precisely align with other data you have, such as stream shapefiles. This can be due to low raster resolution and/or limited control points, or even changing stream directions over time.
  5. Create a pour point for your area of interest, i.e. the lowest elevation grid cell in the area:
    1. Examine the flow accumulation raster to try and find the lowest point (largest value).
    2. In the toolbar Draw, click on the menu next to the rectangle, and choose the Marker tool. Then click on the grid cell to be used as the pour point.
    3. The point you choose should now be selected; in the menu Drawing choose Convert Graphics to Features…, give it an output location, e.g. in the file geodatabase hadleyreservoir.gdb with a name such as pour_point, click on the checkbox  Automatically delete Graphics after conversion, and save the result.
    4. In the window  ArcToolbox, double-click on the tool  Snap Pour Points to create a raster with the pour point(s) in the best location.
  6. Determine the watersheds:
    1. In the window  ArcToolbox, double-click on the tool  Watershed and fill it in.

  7. You can determine the area of the watershed by looking at its dialog Properties and tab Symbology; by default the watershed will be colored by unique values, and the number of ”1”s present is the number of pixels.

    Then switch to the tab Source where you can determine the raster’s Cell Size and Linear Unit.

    For the Hadley reservoir watershed, that’s 102,586 pixels, and each pixel is 8 ft square, resulting in an area of 102,586 x (8 ft / 5280 ft/mi)2 = 0.2355 mi2.

Georeferencing Scanned Maps

Traditional paper maps contain a great deal of geographic information, so it’s important to be able to incorporate them into GIS.

A Map of Amherst with a View of the College and Mount Pleasant Institution
by Alonzo Gray & Charles B. Adams,
Published May 1833 by
Pendletonís Lithography, Boston, MA.
(Source: The David Rumsey Historical Map Collection,

Paper maps are ubiquitous, and often they contain data that are useful in a GIS map, e.g. as a background for other data or to compare modern features with historical locations.

A paper map must first be scanned into a digital format, a now-common procedure that we won’t go into here.

Scans of paper maps and aerial photos must then be spatially positioned to use them with other GIS data, a process known as georeferencing.

To position the scanned map so that it aligns with other GIS data, we can compare it with known reference points or control points, e.g. from an existing digital map or as collected by a GPS receiver.

At a minimum a scanned map must be moved to its correct geographic position, oriented properly, and scaled to its correct size; this requires at least two control points.

Sometimes traditional maps are distorted; this might be due to:

  • Poor measurement;
  • Intentional focus on the relative position of features;
  • Non-vertical perspective, e.g. in aerial photos and panoramic maps;
  • Unknown projection.

Such distortions will likely require a non-uniform scaling to align with known features; this requires at least six control points.

Procedure 7: Georeferencing a Scanned Map

For this procedure you must already have a scanned map available, e.g. the 1833 map of Amherst shown above. You must also have some reference layers for comparison, such as boundary files, orthophotos, or GPS points.

  1. Begin by adding the reference layer(s) and scanned map toArcMap Icon ArcMap:
    1. Add one or more reference layers for comparison, e.g.  amherst_boundary.lyrand  amherst_2004.sid (see Constructing and Sharing Maps for details).
    2. If you know or can guess the projection of the scanned map, change the spatial reference of the map to match (see Mapping Geographic Coordinate Data for details). Otherwise, if you don’t want to match the reference layer(s), a good option is Mercator, since it is shape-preserving and also orients north upward, a common characteristic of paper maps.
    3. Add the scanned map, e.g.  amherst1833.sid.
    4. In the dialog ArcMap, you will be advised that One or more layers is missing spatial reference information…; click on the button OK.
    5. Because the scanned map has no spatial reference information, it will be positioned at the origin of coordinates, typically far from the reference layer(s).
      • Optional Step: In the toolbar Tools, click on the button  Full Extent. Viewing the full extent of the data will likely produce two widely separated specks, one the correctly positioned reference layer(s) and the other the unplaced scanned map. Can you tell which is which?
    6. To view the scanned map, right-click on its name in the Table of Contents and then click on the menu item  Zoom To Layer.
    7. Examine the added map and get a good idea of its extent and any marked boundaries.
    8. Return to the original location by right-clicking on a reference layer’s name in the Table of Contents and then clicking on the menu item  Zoom To Layer.
    9. Zoom in or out from the reference layer so that its recognizable features roughly match those of the scanned map.
  2. Now initiate the georeferencing process:
    1. If the Georeferencing Toolbar is not already visible, click on the menu View, then point at the menu item Toolbars, then click on the menu item Georeferencing. After the toolbar appears, you can dock it out of the way, by clicking-and-dragging it anywhere around the window frames.
    2. In the toolbar Georeferencing, click on the menu Layer:, then click on the menu item for the scanned map (if isn’t already selected — ArcGIS will list all image layers without a spatial reference, and more than likely this will be the only one).
    3. Click on the menu Georeferencing, and then click on the menu item Fit to Display. The result will look something like the image at the right.
    4. This is a good time to save your map; in the toolbar Standard, click on the button  Save.
  3. You must now add a control point that links the same recognizable location on the two layers, by first clicking on it on the scanned map, and second clicking on it on the reference layer.
    1. Locations on the scanned map are recognizable in a number of ways:
      • Point features are typically labeled;
      • Linear features such as streets, railroads, rivers, canals, and political boundaries are usually labeled and have intersections or sharp corners;
      • Survey markers will often have explicit coordinates printed next to them;
      • A graticule will have intersections of meridians and parallels and explicit coordinates at the map edges.

      In the last two cases it’s usually easiest to guess a coordinate location on the reference map and then correct it later, as described below. Warning: to use such coordinates you must be working in the spatial reference of the scanned map!

    2. When you have identified a location on both maps, in the toolbar Tools, click on the button  Zoom In, and then click and drag across both layers to draw a rectangle containing this location on both maps.
    3. If you can’t clearly distinguish this location on the scanned map, drag another small rectangle around it to zoom in further.
    4. In the toolbar Georeferencing, click on the button  Add Control Points.
    5. In the scanned map, click on this recognizable location.
    6. If you’ve made a mistake, you can hit the key Escape to stop the link, and then continue with Step (i).
    7. If you zoomed in a second time in Step (b), then in the toolbar Tools click on the button  Go Back To Previous Extent.
    8. If you can’t clearly distinguish the recognizable location on the reference layer:
      1. In the toolbar Tools, click on the button  Zoom In, and drag another small rectangle around it to zoom in further.
      2. In the toolbar Georeferencing, click on the button  Add Control Points. Notice that it still remembers that you have already initiated a control point by clicking on the scanned map.
    9. In the reference layer, click on the recognizable location; you’ll notice that the cursor will snap to feature vertices and end points.

      The scanned map will now shift its position to bring the two points into alignment.
    10. In the toolbar Tools, click on the button  Go Back To Previous Extent to return to the overview.
  4. Repeat Step 3 with a second recognizable location; this will uniformly scale and rotate the map to align both the first and second points.
  5. Repeat Step 3 a third time using a point that’s widely separated from the line connecting the first two points. This will nonuniformly scale the map and rotate it to align all three points. This is called a first-order polynomial (affine) transformation.
  6. After a fourth application of Step 3, most likely the two points linking the ends of the control point will no longer be perfectly aligned, having some residual distance represented by a blue line, as seen to the left. This is because there are no additional free parameters in this transformation, and a best fit must be calculated.
  7. For most applications you will want to repeat Step 3 several more times, using points around the edge and then throughout the middle of the area of interest.
  8. To snap to intersections between two different layers, such as the boundary and street layers, it’s necessary to turn on that very useful feature; menu Customize > Toolbars > Snapping; dock the toolbar, and then menu Snapping > Intersection Snapping.
  9. A full description of the control points you have set up is provided in the Link Table.
    1. In the toolbar Georeferencing, click on the button  View Link Table.

      The dialog Link Table should now appear, listing each control point link and their starting (Source) and ending (Map) locations.
    2. If you click on any control point link in the table, it will also be highlighted in yellow on the map.
    3. The link table provides information about the residual distance between between the two ends of a control point link, and the Total RMS Error, an average of the residuals, which describes how far out of alignment the entire transformation is. We would like it to be as small as possible. Comparing individual residual distances to the total RMS error can indicate which control points are unusually separated. This might be due to:
      • poor surveying;
      • rerouting of roads, railroads, or canals, and the meandering of rivers;
      • deliberate abstractions, e.g. the separation of features to make them more distinguishable;
      • bad GPS readings;
      • accidental clicks.

      These points can be removed from consideration by clicking them in the table and pressing the key Delete.

    4. The X and Y values in the Link Table are editable; this is most useful if the control points are survey markers or graticule intersections whose values are printed on the map, and can be typed into the fields XMap and YMap.
    5. Warning: ArcMap does not store information about the link table, so to be able to return to where you left off after quitting or to restore from a crash, you should periodically save your table by clicking on the button Save… . This will let you create a text file storing your control points that can be reloaded later by clicking on the button Load… .
    6. Click on the button OK to dismiss the Link Table dialog.
  10. Another way to improve the fit is to use nonlinear transformations. Their effect on the scanned map may not always be desirable (for example, you wouldn’t use them on a presumably accurate map that merely needs to be positioned). There are several options available:
    1. In the toolbar Georeferencing, click on the button  View Link Table.
    2. In the dialog Link Table, click on the menu Transformation:, and then click on one of the following menu items:
      • If you have at least six control points, the item 2nd Order Polynomial becomes available. With exactly six, the Total RMS Error will be zero.
      • If you have at least ten control points, an additional option is 3rd Order Polynomial . With exactly ten, the Total RMS Error will be zero.
      • Also available with at least ten control points is the option Spline. It provides an exact fit for all additional control points, but can be very slow due to the large number of calculations required.
      • An option available at all levels is Adjust; it is very fast for even hundreds of control points, but produces discontinuities in the image at the points’ exterior boundary.
    3. Click on the button OK to dismiss this dialog.
  11. Once you’re satisfied with the fit of the transformed map, you can save it as a new raster layer for later use. Be aware that this process can take a while for a large map. If you haven’t already done so, it’s advisable to disable background processing.
    1. It’s a good idea to first save your control points as described in Step 8(c).
    2. In the dialog Georeferencing, click on the menu item Rectify….
    3. In the dialog Save as, in the text field Output Location:, click on the button  Browse and select the folder (not the file) where you want to save the new raster.
    4. In the menu Format:, choose an output format; for scanned maps, JP2 or JPG is preferred, though PNG can also be good for relatively simple images. JPG is the most compatible with external applications and will typically produce the smallest files if you are willing to sacrifice image quality.
      • If you choose JP2 or JPG, in the text field Compression Quality (1-100): type a value or leave the default (anything less than 100 will be lossy).
    5. In the text field Name:, adjust the file name to be more descriptive, e.g. amherst1833rectified.jp2. Don’t change the file extension here, use Step (d) instead. Warning: GRID-format names must have a base that’s less than 13 characters long.
    6. Click on the menu Resample Type:, and then click on one of the menu items Bilinear Interpolation or Cubic Convolution(better but slower). The option Nearest Neighbor is best only for categorical data.
    7. The new raster’s cell size is initially based on that of the scanned map, and it’s usually best to leave it at the default. You can, however, reduce the file size by increasing the cell size, by typing a new value in the text field Cell Size:.
    8. The new raster will be a rectangle in the current coordinate system, and that means that areas outside of the transformed map will be set to NoData. By default this value will be 0 (black), but you can assign those grid cells another value (e.g. 1 — white) by filling in the text field NoData as:.
    9. Click on the button Save.
  12. Now review the rectified image:
    1. Add the rectified image to your map, e.g.  amherst1833rectified.jp2.
    2. The NoData areas can be made transparent as follows:
      1. Double-click on the name of the rectified image in the Table of Contents to bring up the dialog Layer Properties.
      2. Click on the tab Symbology;
      3. Click on the checkbox  Display Background Value: (R,G,B); leave the default color as No Color.
      4. Click the button OK.
    3. If you use a file format other than JPG or JP2 or PNG, ArcGIS automatically calculates the statistics of the colors in the image, and then uses them to provide what it thinks is a better color display. This is almost always incorrect for an actual image (as opposed to rasters describing quantities like elevation). To turn off the use of statistics for color display:
      1. Double-click on the name of the rectified image in the Table of Contents to bring up the dialog Layer Properties.
      2. Click on the tab Symbology;
      3. In the area Stretch, in the menu Type:, select the menu item None; this allows the image’s colors to be displayed unchanged.
      4. Click the button OK.
    4. in the Table of Contents, click off the checkbox next to the name of the rectified image, e.g.  amherst1833rectified.jp2. You can now see that the rectified image matches the scanned map, the reference layer, and the control points.

Previous: Mapping Coordinate Data

Mapping Raster Data

Following: Editing Vector Data

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