Operating a GPS Unit (GeoExplorer I/II)

MA Department of Environmental Management GIS, March 2001

•	What is GPS?
•	The GPS unit
•	Data dictionaries
•	Collecting data in the field
•	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—we will eventually move to GeoExplorer3) 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 runs on either a large lead camcorder battery or 4 AA batteries.  The camcorder battery needs to be recharged but lasts longer, while the AA batteries only last a few hours but are much lighter.  

AA batteries go into a small black pack which is inserted into the back of the unit.  To remove the pack, you need to press the small plastic lever/button on the bottom of the unit.  Add new batteries and push the pack into the unit.  Make sure you bring extra batteries when you go out into the field!

If you use a camcorder battery, you need to connect it to the unit with a separate pack (not the one that holds AA batteries) that contains one lithium battery and slides into the same spot as the AA battery pack.  There is a hole at the bottom of this pack; plug the spiral cord into this hole.  This end of the cord is by far the weakest part of the unit; make sure you thread the cord through the plastic cord stabilizers because the cord breaks very easily.  The other end of this cord looks like it can plug into a car cigarette lighter; it plugs into another very short cord that attaches to the large black camcorder battery.  The camcorder battery can be recharged using another set of cords that plug into a wall outlet.

There is a second spiral cord that plugs into the round 8-hole plug on the back of the unit and into the I/O port on your computer.  This cord is used for downloading your data onto your computer using Pathfinder Office software once you are done collecting.  Some users may just send the unit back to the Boston office for downloading the data, so you may not need this cord.

The GeoExplorer I/II unit has 8 buttons.  There is an Esc (escape) button, four arrow buttons, an “Enter” button (the one with a diamond, in the middle of the arrow buttons), a power button (the lowest one) and a button with a curved arrow (which you will almost never use).  You will navigate through the menu system using the arrow keys, the Enter key, and the Esc key.  Generally, you select an option using the up and down arrow keys, and choose it using the Enter key.  The Esc key is used for backing up or canceling an action. 

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 power button to turn on the unit.  After a moment, the Main Menu will appear.

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

We will be collecting points and lines, which will be stored in a rover file.  To open a new rover file, move through the choices of the main menu using the up- and down-arrow keys until “Data Capture” is blinking.  Press the Enter key (the one in the middle with the yellow diamond).  Then, choose “Open Rov. File” using the same keys.  Now you have started a new file!  The name of the file will be on the top line of the 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, go to the File Status option and look at the Free: number.  The total memory the unit has is about 192K; if the majority of this has been used, you may wish to 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, go to Main Menu Data Capture Delete File, 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 choose “Select Feature.”  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.

Now you are collecting a point feature.  The number in the upper 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 pressing the up and down arrows, and move along the line using the left and right arrows (keep your comments short, since it takes a long time to type them in this way!).  Again, press Enter when done.  Number attributes work in a similar way.  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 upper 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 choose Close 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.  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 break (or every two 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, choose “Close File” (you may have to press Esc to get to the menu with this choice).  When you are finished collecting data you must close the rover file or your data may be lost!

To shut off the unit, hold the Power button (the lowest button, the one with the line through it) down for five 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.

Troubleshooting

There are many things that can go wrong when using the GPS unit.  Many of these are discussed in the little yellow booklet 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 upper right corner (it will be 0 if you are not getting readings), and there should be the name of a feature in the upper left corner (like “Point” or “Trail,” etc.).
2.	Make sure the unit is not paused.  There should be a menu item that says Pause (if it is there, you are not paused).  If there is a menu item that says Resume, choose it and it should switch to Pause.
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 the Power button for LESS than five seconds—if a capital “A” appears on the bottom line of the screen, you are OK.  If a lowercase “a” appears, you need to let the unit collect the almanac.  Just hold 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.  You can see how many satellites are visible by choosing Main Menu, then GPS Status, then Sat. Tracking.  This screen will show you details about the satellites in the sky.  Go back to Main Menu, then Data Capture 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.  Go to Main Menu Configuration Rover Options Pos Mode, press Enter, and change it from Manual 3D to Auto 2D/3D.  
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.  You can see what your PDOP is on the Sat. Tracking screen mentioned above.  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 Main Menu, then to Configuration, then Rover Options, then PDOP Mask.  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.  We do not currently have an external antenna.  GeoExplorer “I” units cannot be attached to an external antenna; later models can.

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 the Main Menu Configuration Rover Options Feature Logging Line/Area, pressing Enter, and changing the number of seconds.  Five second intervals are recommended.  If you are on foot, collecting line readings more often than every 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.

The unit won’t turn on or it suddenly turns itself off
Assuming you have the batteries connected correctly, this probably means that the batteries are dead.  

If you are using an external camcorder battery, you need to recharge it or switch to AA batteries temporarily (if you have a AA battery pack).  The pack on the back of the unit that the battery cord plugs into contains an alkaline battery that should keep the unit’s memory from being deleted even if you remove the camcorder battery for recharging.  You can feel free to remove the camcorder battery from the cords that connect it to the unit if you need to change the battery.

If you are using 4 AA batteries, you’ll need to replace them with 4 fresh AA batteries.  There is usually enough power in the “dead” AA batteries to keep the unit’s memory alive as long as you keep the battery pack in the unit.  It is recommended that when you do switch the batteries, that you do it quickly.  In our experience, the memory will remain intact with no power for a couple of minutes while you switch the batteries, but if you leave the unit with no pack in it at all for a long time, all the memory will be wiped out.  Do not remove the AA battery pack from the GPS unit unless you are prepared to replace it almost immediately!