Operating a GPS Unit [GeoExplorer 3]

MA Department of Environmental Management GIS, July 2001

•	What is GPS?
•	The GPS unit
•	Data dictionaries
•	Collecting data in the field
•	Navigation
•	Troubleshooting

What is GPS?

GPS, the Geographic Positioning System, consists of 24 US military satellites that send out signals that a GPS unit can use to determine its position on the earth’s surface with surprising accuracy.  The handheld GPS units we will use (Trimble GeoExplorer/Geoexplorer II/GeoExplorer 3) store this data, which can later be downloaded into a GIS system for mapping.  Your unit needs to be able to read signals from at least 3 (and preferably more) satellites at once to work.  

The GPS unit

The GeoExplorer 3 (there is a separate document for GeoExplorer/GeoExplorer II) runs on an internal battery.  The battery lasts for quite a few hours before needing to be recharged.  The unit is self-contained.  All you need to bring into the field is the unit itself (and this document!).

The GeoExplorer 3 unit has 13 buttons.  There is a CLOSE button, an OPTION button, four arrow buttons, an ENTER button, white SYS, DATA, and NAV buttons, a Function (Fn) button, a LOG/Pause button, and a power button (the one with a circle with a line through it).  You will navigate through the different pages using the SYS, DATA, and NAV buttons, and move through menu choices using the arrow buttons and the ENTER and CLOSE buttons.  Generally, you select an option using the up and down arrow keys, and choose it using the ENTER key.  The CLOSE key is used for backing up or canceling an action.  The OPTION button is used to bring up additional context-sensitive options.

You can optionally use an external antenna with the GeoExplorer 3.  The antenna has a long cord with a gold colored plug at one end and a small black magnetic antenna at the other end.  To plug it in, you need the tiny Allen wrench (L-shaped metal stick) that came with the unit.  At the top left of the unit, on the side, there is a small black circular plug that you can remove with the Allen wrench to expose the plug hole.  Plug the antenna in there.  You should be able to get better reception with the external antenna than with the internal antenna.

Data Dictionaries

A Data Dictionary allows you to collect specific kinds of data when you are out in the field.  It consists of several different feature types, each of which can have several attributes.  For instance, you could have a feature called “Gates,” which you would use to record the location of all the gates in a park.  Each gate might have several attributes, such as construction material and lock number, which would be associated with the gate in a database.  You could have a second feature named “Trails,” which would be a line feature that you would use to map out the trails in a park.  Each trail segment could have attributes such as width, construction material, and condition.  In this way, you can collect specific data easily with the GPS unit.

This document will not discuss Data Dictionary creation, since the units you use should have a Data Dictionary already loaded on them.

Collecting Data in the Field

Press the black power button to turn on the unit.  

For instructions on updating the Almanac, checking PDOP and number of satellites, etc., see the Troubleshooting section.

We will be collecting points and lines (and possibly polygons), which will be stored in a rover file.  To open a new rover file, press the DATA key until the DATA File tab is at the front.  Make sure the correct Dictionary is selected (you can change it by moving the cursor down to that line and pressing ENTER).  Move the cursor until Create New File is highlighted and press ENTER.  Now you have started a new file!  The name of the file will be the name that was after the word “File” on the previous screen.  It starts with “R” (for Rover) and the rest of the filename represents today’s date and time.  

One important thing to do before collecting data is to make sure there is plenty of room in the unit’s memory.  If the unit’s memory fills up completely, you can lose data in the file you are working on.  To check remaining memory, press SYS until the SYS Status screen comes to the front.  In the middle of the screen the unit will indicate how much memory (out of 1 MB) has been used up.  If the unit is more than 2/3 full, delete some files.  Do not delete files that have not been downloaded onto a computer!!!  When you get the unit from the GIS staff, its memory should be empty.  If there are files you are positive have been downloaded and can safely be deleted, press DATA until the DATA File tab comes to the front, press OPTION, and choose Delete File(s).  Choose the file you want to delete, and press ENTER.  Choose Yes and press ENTER again.  Once you do this you cannot get the file back, so make sure you know what you are doing.

Now let’s collect a point feature.  Walk to a place you want to record as a point (for instance a trail intersection) and make sure you are on the New Feature screen (press DATA if you are not already there).  You will be presented with a list of features, which may be point, line, or polygon features (there may be more than one of any given type).  Choose the feature you want to record, making sure it is a point feature type (you should be able to tell this from the name—“Other_pt” is probably a point feature, whereas “Trail” would probably be a line type.  Press ENTER.

Now you are collecting a point feature.  The number in the lower right corner indicates how many satellite readings have been taken.  Normally you should take at least 30 readings for a point feature (each reading is a point; the 30 points will be averaged to get a more accurate reading).  If the number of points is growing, you can start entering attributes.

Attributes are pieces of data that are stored with the point or line or polygon you are recording.  They can include text, numbers, or choices from a menu.  Once you have started recording a point, the attributes will appear on the screen and you can scroll between them using the up and down arrow keys.  When you get to one you want to edit, push the ENTER key.  If it is a menu choice attribute, you will be presented with a list of possible values; scroll among them and press the ENTER key when you get to the one you want.  If the attribute is a text attribute, you can choose letters by moving around the on-screen keyboard using the arrow keys and pressing ENTER to add a letter (keep your comments short, since it takes a fairly long time to type them in this way!).  Number attributes work in a similar way, using the arrow keys.  All these attributes are stored with the point (or line or polygon) and serve to identify it and provide extra information about it.

Once you have entered all the attributes you want, and you have taken enough point readings, scroll to “Close Feature” and press ENTER.  Make sure you close the feature before you walk away from where you are standing!

Now let’s take a line feature.  Stand at the beginning of the line you want to record (for instance, a segment of a trail) and choose a line feature.  The user interface will be similar to taking a point feature, but in this case, you will be walking (or riding a bike or riding in a car).  Make sure the unit takes a point (the number in the lower right hand corner will increment) before you start walking, and as you are walking, make sure it doesn’t go too long without taking a point—if it does, stop until it takes one.  It is typically set to take a point every 5 seconds (this can be changed—see Troubleshooting).  If you come to a sharp corner in your line, make sure you get a point reading right at the corner so the shape of the line comes out right.  When you get to the end, make sure you get a point right at the end, then press CLOSE to close the feature (assuming you have finished entering the attributes).

Polygon features are the same as line features, except you must start and end at the same point (actually, the computer will just connect your first and last points with a line).  Often, even if you want to record a feature that is a closed shape, you may be better off recording it as a series of lines (representing the perimeter of the area) or even just points that represent the corners.  These can be made into a polygon later on the computer.  If you do use a polygon feature, you have to close the line—you can’t do half the polygon, stop, and do half later.

Once you have taken all the features you need (or all you want to do in a row before you take a break) you should close the rover file.  This is important to do, since it makes your data safer.  You probably don’t want to put every single point or line in its own file (unless the points are miles apart) but you shouldn’t go for eight hours on one rover file.  A rule of thumb is to close a rover file whenever you take a long break (or every several hours or so), and start a new one when you are ready to continue working.  When you take a break, you should turn the unit’s power off to save batteries.  To close the file, simply press CLOSE when you are on the New Feature page (you have to close your last feature to get there).  When you are finished collecting data you must close the rover file or your data may be lost!  Don’t leave a rover file open overnight.

To shut off the unit, hold the Power button (the button on the lower right, the one with a circle with a line through it) down for a few seconds.

When you are done collecting data, return the unit to GIS staff and the data will be loaded onto the computer, processed, and mapped.

Navigation

The Geoexplorer 3 can navigate to a known position by telling the user which direction to go and how far.  Known positions can be features you have taken previously or waypoints.  Waypoints can be uploaded from a computer or entered manually if you know the latitude and longitude.

Press the NAV button to access the navigation screens (there are two—repeatedly press NAV to see them both).  The screen with the compass works in two ways.  There is a compass in the GPS unit that is only accurate enough to show eight directions (N, NE, E, SE, S, SW, W, NW), but if you are walking fast enough, it can improve its accuracy by using the change in GPS position.  The other screen is the one with the person on the road.  Both screens should show you the Distance to the target and a diamond-shaped road sign that tells you what direction to walk in at the bottom of the screen (if these are not there, you can add them by pressing OPTION and choosing Info Windows, then checking off the Distance and Road Sign checkboxes).  

To start navigating to a point, press OPTION and choose Select Target.  Then choose the point you want to navigate to from the list below (or enter a point by its long/lat value by choosing New—don’t do this unless you are sure you have the coordinates, as it won’t let you cancel without entering them!).  If you are navigating to a waypoint that was uploaded from a GIS computer, they should all be named in an easy-to-identify manner.  If you are navigating to a point that you captured on the GPS unit which has not been downloaded to a computer yet, you’ll need to figure out which point it is by its feature name and order in the list.  You can only navigate to a point in the rover file that is currently being edited.  To start editing a rover file that is closed (for instance, one that you closed before lunch), close any open rover files, press DATA until the File tab comes up, and move to Open Selected File.  Press the down arrow once to go to File: and press ENTER, then choose the file you want to re-open.  Then press ENTER to open the file.  Now press NAV and then OPTION and choose Select Target, then scroll down the list of waypoints until you see the feature names of points from the rover file.  If you don’t know which is which (since they are only labeled with their feature name) you can press DATA and choose the features and look at their attributes to help you remember, then choose them from the waypoint list by remembering their order in the list.

Once you have chosen a waypoint, the GPS unit will tell you which way to walk and how far you have to go.  It is only so accurate; when you get within about 20 feet of the point, it’ll keep sending you in circles (the distance will never equal zero).

Troubleshooting

There are many things that can go wrong when using the GPS unit.  Many of these are discussed in the little laminated sheet included with the unit, but some frequent problems are mentioned here:

I’m not getting any point readings when I try to record a feature
There are many different reasons this can happen.  Check the following:
1.	Make sure you are taking a feature.  There should be a number in the lower right corner (it will be 0 if you are not getting readings), and there should be the name of a feature at the top of the screen, below the folder tabs (like “Point” or “Trail,” etc.).
2.	Make sure the unit is not paused.  If there is a blinking “pause” symbol (two thick vertical lines, like the pause button of a CD player) in the lower right hand corner, push the LOG button to un-pause the feature.
3.	Make sure you have an up-to-date Almanac, which contains satellite information that helps the unit track satellite positions.  If the unit has not been used recently, it may need to collect the Almanac information from a satellite.  You can check if the Almanac is OK by pressing SYS until the SYS GPS tab is at the front. Then press OPTION and choose Advanced Mode.  At the bottom of the screen it will give the date of the Almanac.  If this data is well within the last month, you should be OK.  If it is older, you may need to collect a new almanac.  Just hold or lay the unit out in the open for about 15 minutes and it will load the almanac from a satellite automatically.
4.	Something may be blocking the satellite signals.  This could be anything: tree canopy cover, buildings, hills, or your body.  Try holding the unit above your head, or move slightly away from trees, etc.  The unit shows you the number of satellites it is receiving data from in the upper-right-hand corner, next to the satellite icon.  You can see which satellites are visible by pressing SYS until the SYS GPS tab comes to the front.  This screen will show you details about the satellites in the sky.  Press  DATA to return to collecting your feature.  It may be impossible to collect signals in some locations.  In the worst case scenario you may need to collect a point nearby and make a note that the point is (i.e.) 20 meters to the North of where the point should be.  
5.	If the unit is set to collect only 3D data, you can set it to allow 2D data.  You need at least 4 satellites to collect 3-dimensional points (points with an altitude component) and only 3 to collect 2-dimensional points.  If you are having trouble receiving from more than 3 satellites at once, you can change this option.  Press the SYS tab until the Setup tab comes to the front.  Select Configurations, then GPS.  This will take you to the GPS Precision screen.  Move the cursor across to the Custom box (If there is no “Custom” box, press Option and choose Advanced Mode) and then move it down to the Minimum Satellites line.  Press ENTER and change the number to “3”.  Only do this if you are consistently getting three satellites but are unable to get reception from four.  Make sure you change it back to four once your reception improves; data collected this way is of better quality.
6.	Your PDOP may be high.  PDOP (Percentage Dilution of Precision) is a numerical measurement of the accuracy of the data you are getting, based on the “constellation,” or arrangement of satellites in the sky.  If you are receiving from enough satellites but still aren’t collecting readings, your PDOP may be too high.  You can see what your PDOP is on the SYS GPS screen mentioned in a section above (in Advanced Mode).  The PDOP should be below 6 if you want to collect quality data.  If the PDOP is higher than that for several minutes and you can’t remedy it with any of the steps above, you may need to wait until the satellites come into a better position (this may take half a hour or so).  If the PDOP is just over 6 (less than 10) you can raise the maximum PDOP by going to the GPS Precision screen (mentioned above in the 2D/3D section—use Advanced Mode).  Raise the PDOP mask to 8 or 10, and see if you get readings.  Make sure you change the PDOP Mask back to 6 as soon as possible!  The data you collect will not be as accurate with the PDOP mask above 6, and this is not especially recommended.  
7.	It is possible to use an external antenna with the unit; this will result in better data collection, since you will be able to receive data from more satellites.  Ask GIS staff about using an external antenna.

The unit is taking readings too slow/fast when I’m taking a line/polygon feature
You can change how frequently the unit collects readings by going to SYS Setup screen, choosing Feature Settings, choosing your data dictionary, choosing a feature, and changing the Interval.  Five second intervals are recommended for lines and polygons.  If you are on foot, collecting line readings more often than every four or five seconds will result in a very dense line that will require us to delete most of the points manually.  If you are travelling fast (on a bike or in a car) you may wish to change to three seconds.  Feature Logging Points should always be set to one second so you don’t waste time standing around.

The unit won’t turn on or it suddenly turns itself off
This probably means that the batteries have been drained.  You’ll have to recharge the unit back at the office.