GPS to GIS Preparing to Collect GPS Data

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Jump to... You will Need... ~ Preparing the GPS Receiver ~ Preparing your Students ~ Some GPS Tips.

You will Need...

  • To think about useful ways to use GIS to map and analyze the features you wish to map

  • A Global Positioning System receiver (inexpensive models are great, but if you need very accurate data, or if you are collecting large amounts of data, you may need to invest in a more expensive model)

  • Microsoft Excel or another program that can generate database or delimited text files (instructions are given here for Excel)

  • GIS Software (instructions here are for ArcGIS 8.X, ArcView 3.X and ArcExplorer). If you wish to create raster files with your data, you will need the Spatial Analyst extension for ArcGIS or ArcView.

  • Field Data Sheets (a sample is provided here. You can print these off to use in the field). We recommend collecting data on paper even if you have a GPS that will store data points. This is for two reasons: electronic equipment can fail, and students learn important lessons in recording the data that they don't get by just pushing buttons.

  • Clipboards and pencils.

  • Good weather is not required, but it sure helps.

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Preparing the GPS Receiver

It is helpful to aquaint yourself with the GPS system and sources of error before you begin. Trimble, Inc., provides an excellent online tutorial (http://www.trimble.com/gps/; requires Shockwave).

The GPS unit receives transmissions from satellites overhead and triangulates its position based on that information. The more satellites the unit can “see” from its position on Earth, the more accurate its read-out of latitude, longitude, and elevation will be.

Set your GPS units to the units of measurement that you prefer. Consult the documentation that came with your unit to determine how to do this. You may be able to collect data in either meters (also called "xy") or latitude/longitude (instructions are given here for lat/lon). If you will be collecting latitude/longitude data in decimal minutes or degrees-minutes-seconds (dms), you will need to convert the coordinates to decimal degrees in order to map data in a GIS. See this page for instructions on using MS Excel to convert your data.

Set your GPS units to the coordinate system and datum you prefer. Consult the documentation that came with your unit to determine how to do this. For most uses, you will want to use a geographic coordinate system ("unprojected") or Universal Transverse Mercator (UTM), zone 19 north.

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Preparing for the Field

Each group should have a GPS receiver, a data sheet, a clipboard, and a pencil. Your instructor may assign each group a single point, line, or polygon layer and demonstrate how to collect the data and fill out a data sheet. A z coordinate can include height, elevation, vegetation type codes, or any characteristic that varies from place to place and can be expressed in numeric form. Other attributes can be numbers, text, or a combination of the two.

BEFORE you set out into the field, take a few minutes to develop a data dictionary and a data model. For instance, if your group is collecting a point layer of tree species and tree heights, you will first want to establish a code or an abbreviation for each species you will encounter (and leave room for the unexpected!). For a polygon layer of buildings, the data dictionary could include codes or fields for building type, uses, owner, address, etc. (Hint: CAD drawings or blueprints of school buildings can provide building heights. These can be added to your GPS data to build 3D maps of your campus buildings.)

Plan to collect point, line and polygon features on separate data sheets, and collect them in the order that they should appear when they are mapped. In other words, if you want a road to appear as a straight line, walk down the road from one end to the other to collect the points. If you collect points out of order, your nice straight road will appear as a crazy zigzag on your map, depending on the tools you use to generate your lines. The same goes for a polygon; walk around the polygon and collect the points as you go.

Remember to watch for cars and other dangers and to respect private property and the privacy of individuals while you collect your data.

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Some GPS Tips

Each GPS has some way of indicating how accurate the position is at any given time, giving you the number of satellites it is picking up and a range of accuracy in feet or in meters. Your unit may allow you to average your readings over time. Be sure you understand how to read the accuracy of the data and to obtain the most accurate readings possible. Read the manual for recommendations on how to collect the most precise data.

There are several ways you can increase the accuracy of your data: Stand still for a minute or more before taking a reading and average your position if your GPS unit will do averages. If you cannot stop and stand, move as slowly as possible. Thick trees and buildings may block or reflect the signal of satellites overhead-- if possible avoid them, and if you must use your GPS among trees or buildings, average your position over longer periods of time or consider using an external antenna.

If you need more tips, check out our GPS Links page or contact us with specific questions.

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