The Global Positioning System (GPS) provides a quick
means to establish one's location on the surface
of the Earth, to relatively high precision.
The Global Positioning System
is a network of satellites that broadcast location
information.
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.
GPS receivers can be quite
small and compact, but have enough controls to
be a bit complex.
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.
- Walk to an area with as clear a view of the sky as possible.
- Turn on the GPS receiver by holding down the
button
Power (on
the right side, adjacent to the “eTrex” logo),
until the display activates.
- The
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.
- Switch
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).
- 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.
- Walk to an area with as clear a view of the
sky as possible.
- 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.
- The
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.
- Switch
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).
- 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.
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.
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.
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).
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.
- Press the button Find (on the left
side of the GPS receiver, adjacent to the “eTrex” logo
at the bottom).
- In the menu Find,
you'll see a number of "findable" items.
Navigate to the item Waypoints and
select it.
- In the menu Waypoints,
navigate to the item Nearest and select
it.
- A list of nearest waypoints in the receiver will appear.
Navigate to the screen button Menu, and
select it.
- A menu of options will
appear; navigate to the item Delete Alland
select it.
- In the dialog Do you really want
to delete all waypoints?,
select the item Yes.
- Press the button Find (on the
lower left side of the GPS receiver).
- A list of "findable" items will
now appear.
Navigate to the item Waypoints
and select it.
- A list of all waypoints in the receiver will appear.
Press the button Menu (on the
lower right side of the GPS receiver).
- A menu of options will appear;
select the item Delete….
- In the menu Used Symbols, select
the item All Symbols.
- In the dialog Do you really want to delete
all waypoints?,
select the item Yes.
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.
- 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.
- A
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.
To use your GPS recorded positions
in GIS, you must download them to a computer.
Your GPS receiver comes with a cable that
will connect it to a computer's interface
port.
- The
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.
- 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.
- Turn on the GPS receiver.
- The
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.
- 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.
- 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.
- 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.
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.
- Click on the menu Start.
- 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.
- After the application EasyGPS has
finished starting up, click on the
menu Edit,
and then select Preferences....
- 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:
- Adding the Garmin eTrex Legend:
- Click
on the button Add GPS...;
- In the dialog Add GPS,
in the list Manufacturers:,
click on Garmin;
- In the
list Models:,
scroll to eTrex Legend and
click on it;
- Click on the button OK.
- In the
dialog GPS Settings,
the default connection COM1 at 9600 should
be correct, so click on the
button OK.
- Adding the Garmin GPSMAP 60Cx:
- Click
on the button Add GPS...;
- In the dialog Add GPS,
in the list Manufacturers:,
click on Garmin;
- In the
list Model:,
scroll to GPSMAP 60Cx and
click on it;
- Click on the button OK.
- In the
dialog GPS Settings,
the default connection USB should
be correct, so click on the
button OK.
- Click on the button OK.
- 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.
- If the application EasyGPSisn't
already running, start it up as described
in Procedure
5.
- In the menu GPS,
click on the menu item Receive from GPS....
- In the dialog Receive from GPS,
uncheck everything but Waypoints.
- In the menu GPS Settings,
verify that your GPS receiver is
listed, and if it isn't, select it.
- Click on the
button OK.
A list of your waypoints
will appear, and a map that shows
their relative position will be displayed:
- 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.
- 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.
- 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).
When you want to add GPS data
to ArcGIS, it usually needs a little help
to interpret it.
ArcGIS doesn't (yet) understand the .gpx format,
so you must convert such files into
shapefiles using the free utility EGPS2shp.
- Click on the menu Start.
- 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.
- After
the application EGPS2shp has
finished starting up, in the section
ExpertGPS *.gpx file, click
on the button Open
- In the dialog
Open,
navigate to the .gpx file you saved
previously, and click on the button
Open.
- In the section ESRI
shp file prefix,
click on the button = to
select the same file name prefix.
- Uncheck the checkboxes Line and Area,
leaving only Point.
- 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.
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.
- Click on the menu Start.
- 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.
- 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.
- In the dialog Shapefile Properties,
in the tab XY
Coordinate System, click
on the button Select....
- 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.
- Back in the
dialog Shapefile Properties,
click
on the button OK.
The shapefile is now ready to use with ArcGIS.
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