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Geographic Information Systems

Using a GPS Receiver

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The Global Positioning System (GPS) provides a quick means to establish one's location on the surface of the Earth, to relatively high precision.

Topics

Procedures

  1. Setting Up a GPS Receiver

  2. Deleting Old Waypoints from a GPS Receiver

  3. Recording Your Position With a GPS Receiver

  4. Connecting a GPS Receiver to a Computer

  5. Setting up EasyGPS

  6. Downloading Data With EasyGPS

  7. Using EGPS2shp to Convert a .gpx File Into a Shapefile

  8. Assigning the Spatial Reference of a Shapefile


The Global Positioning System

The Global Positioning System is a network of satellites that broadcast location information.

The Global Positioning System
The Global Positioning System Satellite Network
(U.S. Department of Defense)

The Global Positioning System (GPS) is a network of Earth-orbiting satellites. They broadcast information about time and location that GPS receivers can use to determine your position on the surface of the Earth. Typical accuracy can be measured in tens of meters.

The initial GPS network consisted of twenty-four satellites, orbiting the Earth every twelve hours. Anywhere from four to twelve of them are potentially observable at one time. Signals from at least three satellites are required to establish your position, and the more one can see, the more accurate the measurement. Mountains, buildings, and even trees can block the signals, so the clearer your view of the sky, the better.

The newer WAAS satellites use differential correction from ground stations to substantially improve accuracy, sometimes down to 3 meters! Most current GPS receivers can take advantage of these supplementary signals. Very recent models are also more sensitive and provide sophisticated signal analysis to ensure more uniform accuracy.

The basic information provided by GPS is your latitude and longitude (WGS 84 is the datum). Many GPS receivers also have built-in maps that display your location in real time. They can also record positions, known as waypoints, for later upload to a computer and use by programs such as ArcGIS.

For more information about GPS, visit the Garmin GPS web site.


Using a GPS Receiver

GPS receivers can be quite small and compact, but have enough controls to be a bit complex.


Setting Up a GPS Receiver

Amherst College Academic Technology Services has two kinds of GPS receivers, the Garmin eTrex Legend and the Garmin GPSMAP 60Cx.

The latter is more recent and more accurate, so it's generally preferred if available.

Students, faculty, and staff may check out GPS receivers from the ATS Classroom Equipment Group.

 

Garmin eTrex Legend and Exterior ButtonsProcedure 1a: Setting Up the Garmin eTrex Legend

  1. Walk to an area with as clear a view of the sky as possible.
  2. Turn on the GPS receiver by holding down the button Power (on the right side, adjacent to the “eTrex” logo), until the display activates.
  3. Garmin eTrex Legend Satellite PageThe GPS receiver will initially display some startup information, and then switch to the Satellite Page.

    The satellites visible to the GPS receiver will be displayed, initially as hollow circles. As the GPS receiver locks on to the satellites, the circles turn black, and a numbered signal strength bar appears. When WAAS information is being used, the other satellites’ signal strength bars show a ‘D’ (meaning a differential correction has been made).

    Hold the GPS receiver horizontally or tilted slightly towards you, and wait until at least three satellite locks are made, whereupon your location will be displayed, along with the accuracy of your position.

  4. Garmin eTrex Legend Satellite PageSwitch to the Map Page by pressing the button Page (on the right side of the GPS receiver, adjacent to the globe image at the top).

    A map will now be displayed showing your location as a black triangle. As you move, the GPS receiver will determine the direction you are heading and the triangle will point that way.

    Note the lines on the display, streets (darker) and elevation contours (lighter).

    You may also see numbered dots, representing marked positions (waypoints).
  5. The circle surrounding the triangle represents the accuracy of your position. If you can't see it, you are probably zoomed way out to a small scale, or zoomed way inside of it.

    You can change the map scale by pressing one of the buttons Zoom In or Zoom Out (on the left side of the GPS receiver, adjacent to the globe image at the top). Hold them to zoom more rapidly.

 

 

Garmin eTrex Legend and Exterior ButtonsProcedure 1b: Setting Up the Garmin GPSMAP 60Cx

  1. Walk to an area with as clear a view of the sky as possible.
  2. Turn on the GPS receiver by holding down the button Power (on the top side, adjacent to the “GPSMAP 60Cx” logo), until the display activates.
  3. Garmin eTrex Legend Satellite PageThe GPS receiver will initially display some startup information, and then switch to the Satellite Page.

    The satellites visible to the GPS receiver will be displayed, initially as hollow circles. As the GPS receiver locks on to the satellites, the circles turn black, and a numbered signal strength bar appears. When WAAS information is being used, the other satellites’ signal strength bars show a ‘D’ (meaning a differential correction has been made).

    Hold the GPS receiver horizontal or tilted slightly towards you, and wait until at least three satellite locks are made, whereupon your location will be displayed, along with the accuracy of your position.

  4. Garmin eTrex Legend Satellite PageSwitch to the Map Page by pressing the button Page (on the lower right side of the GPS receiver).

    A map will now be displayed showing your location as a black triangle. As you move, the GPS receiver will determine the direction you are heading and the triangle will point that way.

    Note the red lines on the display, representing major streets.

    You may also see numbered blue flags, representing marked positions (waypoints).
  5. The circle surrounding the triangle represents the accuracy of your position. If you can't see it, you are probably zoomed way out to a small scale, or zoomed way inside of it.

    You can change the map scale by pressing one of the buttons Zoom In or Zoom Out (on the middle-left and middle-right sides of the GPS receiver). Hold them to zoom more rapidly.

 


Controlling a GPS Receiver

In addition to its external buttons, the GPS receiver is also controlled by selecting options on its display, e.g. screen buttons, menus, lists, etc.

Garmin eTrex Legend and Exterior Buttons Controlling the Garmin eTrex Legend

There are usually several display options available at a time, but only one is selectable, and it's highlighted in black.

To move from one display option to another, push the  Thumb Stick (immediately to the left of the eTrex logo) away from you, towards you, to the left, or to the right.

To select the highlighted display option, depress the Thumb Stick; you must press the Stick straight inward, as any movement to the sides will move to a new option instead.

Sometimes a selection will result in a new page or menu of options; to return to the previous page, either move to the screen button  Quit in the upper right corner and select it, if present; otherwise simply move to the left or right.

Garmin eTrex Legend and Exterior ButtonsControlling the Garmin GPSMAP 60Cx

There are usually several display options available at a time, but only one is selectable, and it's highlighted in yellow.

To move from one display option to another, push the  Rocker (in the bottom middle part of the GPS receiver) up, down, to the left, or to the right.

To select the highlighted display option, press the Entr button (on the bottom right side of the GPS receiver).

Sometimes a selection will result in a new page or menu of options; to return to the previous page, press the Quit button (on the bottom left side of the GPS receiver).

 

 


Deleting Old Waypoints from a GPS Receiver

If you check out a GPS receiver, after you download its data you should delete any waypoints you have created before returing it to the ATS Classroom Equipment Group.

Since users often don't do this, it's a good idea to verify that they are all deleted before you use the unit yourself, to reduce confusion about these older points.

 

Garmin eTrex Legend Satellite PageProcedure 2a: Deleting Old Waypoints from the Garmin eTrex Legend

  1. Press the button Find (on the left side of the GPS receiver, adjacent to the “eTrex” logo at the bottom).
  2. In the menu Find, you'll see a number of "findable" items.

    Navigate to the item Waypoints and select it.
  3. In the menu Waypoints, navigate to the item Nearest and select it.
  4. A list of nearest waypoints in the receiver will appear.

    Navigate to the screen button  Menu, and select it.
  5. A menu of options will appear; navigate to the item Delete Alland select it.
  6. In the dialog Do you really want to delete all waypoints?, select the item Yes.

 

Garmin eTrex Legend Satellite PageProcedure 2b: Deleting Old Waypoints from the Garmin GPSMAP 60Cx

  1. Press the button Find (on the lower left side of the GPS receiver).
  2. A list of "findable" items will now appear.

    Navigate to the item Waypoints and select it.
  3. A list of all waypoints in the receiver will appear.

    Press the button Menu (on the lower right side of the GPS receiver).
  4. A menu of options will appear; select the item Delete….
  5. In the menu Used Symbols, select the item All Symbols.
  6. In the dialog Do you really want to delete all waypoints?, select the item Yes.

 


Recording Your Position With a GPS Receiver

A record of your position in the GPS receiver is known as a waypoint.

As you move around, it's relatively easy to create waypoints for later use.

 

Garmin eTrex Legend and Exterior ButtonsProcedure 3a: Recording Your Position with the Garmin eTrex Legend

  1. Recording your position with a waypoint is a common activity, so you can do it any time, no matter what's currently on the GPS display.

    When you are at a location where you want to record your position, depress and hold the  Thumb Stick until the page Mark Waypoint appears.
  2. Garmin eTrex Legend Mark Waypoint PageA recorded position is called a waypoint. They are numbered sequentially as they are created.
    • To save the waypoint for future reference, select the screen button OK.

      The display will return to the page Map, where a symbol marked with the waypoint number will be visible once you've moved away from that location.
    • To cancel saving the waypoint, select the screen button  Quit.

 

 

Garmin eTrex Legend and Exterior ButtonsProcedure 3b: Recording Your Position with the Garmin GPSMAP 60Cx

  1. Recording your position with a waypoint is a common activity, so you can do it any time, no matter what's currently on the GPS display.

    When you are at a location where you want to record your position, press the button Mark (on the lower left side), and the page Mark Waypoint will appear.
  2. Garmin eTrex Legend Satellite PageA recorded position is called a waypoint. They are numbered sequentially as they are created.
    • To save the waypoint for future reference, select the screen button OK .

      The display will return to the page Map, where a symbol marked with the waypoint number will be visible once you've moved away from that location.
    • To cancel saving the waypoint, press the button Quit.

 

 


Downloading GPS Data

To use your GPS recorded positions in GIS, you must download them to a computer.


Connecting a GPS Receiver to a Computer

Your GPS receiver comes with a cable that will connect it to a computer's interface port.

 

Procedure 4a: Connecting the Garmin eTrex Legend to a Computer

  1. DB-9 ConnectorGarmin eTrex Legend and Exterior ButtonsThe Garmin eTrex Legend uses a standard serial connection, which attaches to a computer using a DB-9 connector like the image on the left.

    On a Windows computer, the software calls this interface port COM1 (or COM2, etc. if there is more than one).

    Locate this connector on the computer, usually on the backside, and carefully plug in the matching end of the GPS receiver's connector cable.
  2. The Garmin eTrex Legend has a special External Data Connector on its back side, hidden under a rubber flap at the top of the receiver. There is a raised ridge off-center that forces an external connector into a single orientation.

    Open the flap, align the other end of the receiver's connector cable with the connector's raised ridge, and carefully slide it into place.
  3. Turn on the GPS receiver.

 

 

Garmin eTrex Legend and Exterior ButtonsProcedure 4b: Connecting the Garmin GPSMAP 60Cx to a Computer

  1. DB-9 ConnectorThe Garmin GPSMAP 60Cx uses a Universal Serial Bus (USB) connection, which attaches to a computer using a USB-A connector like the image on the left.

    Locate this connector on the computer, usually on the backside, but also often on the front side. Avoid connectors on monitors or keyboards, as they often don't provide high speed or sufficient power.

    Carefully plug in the matching end of the GPS receiver's connector cable.
  2. The Garmin GPSMAP 60Cx has a small USB-C connector on its back side, hidden under a rubber flap near the top left corner of the receiver.

    Open the flap, align the other end of the receiver's connector cable with the connector's shape, and carefully plug it in.
  3. Turn on the GPS receiver by holding down the Power button (on the top side, adjacent to the “GPSMAP 60Cx” logo), until the display activates.
  4. At this point, Windows may complain that the USB driver for the GPS receiver is missing; in this case, an administrator will need to add it.

 

 


Downloading GPS Data with EasyGPS

EasyGPS is a free computer application that talks to your GPS receiver and downloads its data to your computer, so that it can be used with ArcGIS. You must first set up EasyGPS so it knows what kind of receiver you have.

Procedure 5: Setting Up EasyGPS

  1. Click on the menu  Start.
  2. Point at the menu item All Programs, and if you are in the public labs point at the menu item  Course-related. Then click on the menu item  EasyGPS.
  3. After the application  EasyGPS has finished starting up, click on the menu Edit, and then select Preferences....
  4. In the dialog Preferences, in the tab My GPS Receivers, in the list Select a GPS receiver, if your model is listed, click on it.

    If it's not listed, you'll need to add it:
    1. Adding the Garmin eTrex Legend:
      1. Click on the button Add GPS...;
      2. In the dialog Add GPS, in the list Manufacturers:, click on Garmin;
      3. In the list Models:, scroll to eTrex Legend and click on it;
      4. Click on the button OK.
      5. In the dialog GPS Settings, the default connection COM1 at 9600 should be correct, so click on the button OK.
    1. Adding the Garmin GPSMAP 60Cx:
      1. Click on the button Add GPS...;
      2. In the dialog Add GPS, in the list Manufacturers:, click on Garmin;
      3. In the list Model:, scroll to GPSMAP 60Cx and click on it;
      4. Click on the button OK.
      5. In the dialog GPS Settings, the default connection USB should be correct, so click on the button OK.
  5. Click on the button OK.
  6. To verify the connection to the GPS receiver, in the menu GPS, click on the menu item Display GPS Information..., which should provide some details about your GPS receiver.

Once EasyGPS recognizes your GPS receiver, it's simple to download its data.

Procedure 6: Downloading Data With EasyGPS

  1. If the application EasyGPSisn't already running, start it up as described in Procedure 5.
  2. In the menu GPS, click on the menu item Receive from GPS....
  3. In the dialog Receive from GPS, uncheck everything but Waypoints.
  4. In the menu GPS Settings, verify that your GPS receiver is listed, and if it isn't, select it.
  5. Click on the button OK.

    A list of your waypoints will appear, and a map that shows their relative position will be displayed:

    EasyGPS window
  6. You can view information about individual waypoints by scrolling to them in the list or clicking on the button  Select, and then right-clicking on them in the list or on the map, respectively. In the pop-up menu that appears, selecting Edit Waypoint.
  7. Review your waypoints to make sure there aren’t any extraneous points, and delete those if necessary, by clicking on them and pressing the Delete key.
  8. Save your data in a new file by clicking on the button  Save, navigating to where you want to store your ArcGIS data, and giving the file an appropriate name.

    EasyGPS saves your files in the open format known as GPS Exchange (.gpx).

 


Importing GPS Data into ArcGIS

When you want to add GPS data to ArcGIS, it usually needs a little help to interpret it.


Converting an EasyGPS .gpx File Into a Shapefile

ArcGIS doesn't (yet) understand the .gpx format, so you must convert such files into shapefiles using the free utility EGPS2shp.

Procedure 7: Using EGPS2shp to Convert a .gpx File Into a Shapefile

  1. EGPS2shp WindowClick on the menu  Start.
  2. Point at the menu item All Programs, and if you are in the public labs point at the menu item  Course-related. Then click on the menu item  EGPS2shp.
  3. After the application  EGPS2shp has finished starting up, in the section ExpertGPS *.gpx file, click on the button  Open
  4. In the dialog Open, navigate to the .gpx file you saved previously, and click on the button Open.
  5. In the section ESRI shp file prefix, click on the button = to select the same file name prefix.
  6. Uncheck the checkboxes Line and Area, leaving only Point.
  7. Click on the button Convert.

    The three files required for a shapefile (.shp, .shx, and .dbf) will be created in the same location as the input .gpx file.

 


Assigning the Spatial Reference of a Shapefile

EasyGPS data (.gpx) files will always be in the fundamental spatial reference of the GPS receiver, geographic coordinates with the datum WGS 84. However, you must specifically tell this to ArcGIS. The ArcCatalog program lets you modify individual data files, and in particular assign them a spatial reference.

Procedure 8: Assigning the Spatial Reference of a Shapefile

  1. Click on the menu  Start.
  2. Point at the menu item All Programs, and if you are in the public labs point at the menu item  Course-related. Then point at the menu item  ArcGIS, and click on the menu item  ArcCatalog.
  3. After the application  ArcCatalog has finished starting up, in the left-side section Catalog, navigate to the shapefile  *.shp, and double-click on it. If necessary, first make a new connection to the folder containing the shapefile.
  4. In the dialog Shapefile Properties, in the tab XY Coordinate System, click on the button Select....
  5. In the dialog Browse for Coordinate System, navigate into the folder  Geographic Coordinate Systems, then into the folder  World, and finally double-click on the file  WGS 1984.prj.
  6. Back in the dialog Shapefile Properties, click on the button OK.

The shapefile is now ready to use with ArcGIS.


Previous: Mapping Coordinate Data

Using a GPS Receiver

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