An Introduction to GPS

The global positioning system (GPS) has been established and operated by the U.S. military as a means of enabling personnel to obtain their location with consistently high accuracy at any place in the world. By making this information resource accessible to anyone, we have a tool which is very useful for a wide array of uses, from ship navigation, to recreational use, to commercial vehicle tracking. The potential applications are very broad and will be increasing rapidly, especially now that Selective Availability (deliberate signal degradation) has been turned off. This has improved the expected accuracy for simple hand-held units to be in the range of five to ten meters instead of the fifty to one hundred meters previously to be expected. Additional data gathering and processing techniques are available which can improve the positional accuracy to the range of one to five meters.

In this assignment you will use a GPS hand-held unit to gather map coordinates for a specified point and for a linear feature in a nearby part of campus. Our GPS units have several other useful features which we will not be using in this assignment. For example, you can use one to navigate to a known or previously visited location, and you can also store attributes (names, values, types, etc) about the locations for which you are collecting positions.

The attached map shows a part of campus to the south and southwest of this building. On it you can see sidewalks, parking areas, lawns, buildings, sports fields and sculptures which make up our visual landscape. A linear feature has been highlighted (the sidewalk around a lawn area, a parking lot, the track, etc) and you are to gather enough coordinate data to produce a map of that feature. Once you have identified your map feature, you will collect points to define its shape, convert this coordinate data into line entities and then prepare a map showing your results. The objective of the map is not only to draw the feature, but also to show how well the shape and location of the feature as you determined it to exist matches the same feature as determined by other established map data. Thus your map will include two versions of the assigned feature:

1) as produced from the coordinates you collected with the GPS unit

2) as produced from the coordinates obtained from the campus planners.

In addition to collecting lines as a series of points to define your geographic feature, you must obtain coordinates for a point location for which we have known coordinates as determined by a surveyor. This will provide a measure of the agreement between a professionally surveyed location and the coordinates you are able to obtain using the GPS technology.

This assignment will take two class sessions, one for assignment introduction and instructions and the second for processing your field data and preparing your map. In the week between the two classes, you will check out one of the GPS units to gather your assigned mapping data and to download this data into a computer file for processing and use during the second class session.

Preparation for collecting data:

The GPS unit has many parameters and features which make it a very versatile and precise instrument for collecting spatial data and feature attributes. For this assignment, these parameters and controls will be preset so that you will not be responsible for these settings. If you were to use one for a more complete data gathering effort for a field research project, you would need more instruction on these settings and their significance. For now, you have to locate the linear feature you are to map, follow instructions on how to use the GPS in the field, and use the facilities in the computer lab to download, process and map your dataset.

Look at the attached map for the feature assigned to you (highlighted). Note the location of the mandatory control point for which everyone must gather coordinates, and plan the time when you want to carry out the field part of the assignment.

Data collection:

Mandatory point location

You will find a circular metal plate in the sidewalk some thirty-five feet north of the intersection of the south-bound sidewalk (going towards Fairhaven) and the east-west sidewalk going over to the Police Station. It is about nine inches in diameter, has a small center hole, and has the letters 'MON' on it. Keeping the instrument high to maintain satellite lock, collect the point for at least 30 seconds. Plan your collecting activity so that you collect this location twice, before and after collecting your line feature.

Linear feature

In LINE mode you will follow the line which will define the feature you have been assigned, e.g. along the edge of a sidewalk, along the curb of the parking area, along the inside (or outside) edge of the track, etc. Note that the GPS unit should be beeping about once each second. Also, the display will indicate how long you have maintained good contact with enough satellites, and the number of points you have gathered for the feature. You can alternate between a GPS screen which displays the location of the satellites and your current computed location (in Longitude and Latitude), and the file display which shows how many points have been recorded for the current feature and how long you have maintained a good lock on the satellites. Pressing the SYS button gives you the first screen and pressing the DATA button shows you the second.

In the field

When you are ready to go outside and begin collecting the data for your features, get clear of the building, turn on the GPS power (press the black button in the lower right of the unit) and note that it begins by stating that sufficient satellites are not available. It will take several minutes (depending on the current satellite configuration and the existence of buildings and/or trees nearby) for the receiver to lock on to enough satellites to begin computing a position. Keep the receiver well above waist level so your body is not blocking its view of part of the sky. While you are waiting for the GPS to initialize, move down the sidewalk towards Fairhaven to find the required control point near the sidewalk intersection. When the display shows your current location at the bottom of the GPS screen, you are ready to begin collecting coordinates. Press the DATA key so you can open up a file for storing your data. The FILE screen allows you to CREATE NEW FILE or to OPEN SELECTED FILE. CREATE NEW FILE should be highlighted as the default choice: press ENTER to choose that option. The default data types which can be added to your new file will be shown as POINT, LINE or POLYGON. Use the up and down arrows (the keys above and below the ENTER key) as needed to highlight the entity type desired (POINT for the monument position, LINE when doing your linear feature). To begin collecting data, press the LOG button (lowest key on the instrument). Collect about a minutes worth of data while standing at the Monument control point, being careful to keep the GPS unit high enough that you don’t shade much of the sky. Use the SYS key if you want to check on the number of satellites currently being used. When you have collected the required number of points (about thirty), change to the DATA screen, if you are not already viewing that screen, and then press the CLOSE key to stop collecting data for that point. Proceed to the starting point for the linear feature you have been assigned. On the FILE screen use the arrow key to switch the feature type to LINE, press ENTER to select that type, and ENTER again to begin collecting coordinates as you walk slowly along the feature. You can alternate between the GPS display (press SYS) and the file display (press DATA) but you need to be on the DATA screen to begin and end data collection for features and to close your file.

Notice that a blinking pencil icon appears in the lower right corner of the GPS screen, indicating that the receiver is recording position data while it also shows how many points have been collected for the current feature. Use the DATA button when you want to return to the file control screen. When you have walked around your line feature and returned to your starting point, Press CLOSE to stop collecting data for the line feature. Return to the control point, change to POINT type and collect your second set of coordinates for that location.

Tips:

All GPS users have the option of clearing out any old files on the GPS unit to be sure of having space for collecting data. To be sure of retaining your data, the simplest procedure is to allow time in your field schedule to download your file(s) immediately after collecting in the field.

Every second the unit will beep, indicating that it has taken a reading. Just above the pencil icon (GPS screen) is displayed the number of points which have been recorded. Walk at a speed which will record the details of changes in direction of the feature.

Use the LOG key on the receiver to stop recording data if you are interrupted while walking your route.

If you lose the signal from the satellites, press the LOG key and wait for the signal to be reconnected. You may need to walk to an open area to recover the signal. Once you regain the signal go back to the line and press LOG to resume collecting. Note that good signal reception is difficult when you are close to buildings and when you are under trees.

Saving and Downloading your file(s):

When you have collected your assigned line feature and the designated control point (before and after the line feature), and have closed your file, you are ready to return to the computer lab and download your file. Turn off the GPS by pressing the black button again, this time hold it down for several seconds until the message on the screen says the power is off. Try to schedule your field time so that you allow a half-hour after the collecting process in order to have time for downloading and storing your data. Processing your field data so that you can convert it into a form for your final map will take place during the second class period.

The file downloading is performed on any of the six lab computers setup with the GPS Support Unit. These six computers are: the computer normally used by an instructor when using the video projector, the four along the wall with the clock on it, and the one closest to the large paper cutter in the corner of the room. Attached to each of these computers is a GPS Support Unit which will establish the physical connection between the GPS unit and the computer, and it also has the software to transfer your file. Login to your lab account and place your GPS unit into the Support Unit. It should begin to display information on the GPS screen. On the computer select the software called Pathfinder Office (this can be done via the desktop icon or by finding it on the Start/Program list of available programs). When the program is loaded, find the Data Transfer command on the Utilities pull-down menu. When you select Data Transfer, the program will try to make a connection to the GPS unit and then will give you a list of the files it finds on that unit. The default file naming in the GPS unit consists of the letter R followed by a six digit number and a letter. The six digits are MMDDHH where the first two digits are the month the file began, the DD are the day in the month and the HH are the hour of the day (based on Standard time on the Greenwich meridian, seven hours ahead of Pacific Daylight, eight ahead of Pacific Standard). Thus a file begun on May 21st at noon would have a file name of R052119A (05 for the month of May, 21 for the 21st day, and 19 for the hour between local noon and 1 pm as converted to GMT). The letter A is assigned to the first file opened during that hour, a B to the second, etc. You should keep a written log of the time you did your collecting, and the file name(s) assigned to you by the GPS unit.

The files will not be very large so a floppy or a zip cartridge is more than adequate for storing your data. Depending on availability of user space, you may also be able to store this in your U: drive, the computer space assigned to you with your campus universal account. Complete your data collection and file downloading before you come to class for our second lab session. During that session you will perform the data export and final map preparation.

If you have questions, look for Gene in AH 24, next to the Spatial Analysis Lab.

Procedures for the second class session:

exporting to ArcView in SPC 27

preparing your final map in ArcView