Using recreational-grade GPS units for fieldwork
GPS technology has greatly simplified the process of locating sites and features on the landscape and mapping positions, linear features, and areas. There are two main groups of GPS units used by field scientists: "recreational" and "mapping" grade GPS units. This post is about using Recreational GPS units for fieldwork in 2006.
Recreational GPS units, such as Garmin and Magellan models, cost between $150 and $500 and they are widely used because they offer reasonably high accuracy for a low price. In areas where WAAS or EGNOS is available (North America and Europe, possibly worldwide in the near future), you can get 2-5m spatial accuracy with these units if they support this correction type (made since ~2001) and WAAS or EGNOS is turned On. Otherwise expect a spatial accuracy of 10-15m per measurement.
This post contains suggestions for field research using recreational GPS units for data gathering.
A. Accuracy
B. Data logging
C. Coordinate Systems
D. Do you need mobile GIS?
A. Accuracy
A major difference in accuracy of recreational grade GPS receivers as opposed to Mapping grade receivers is apparently due to the technology used in their clocks. According to Trimble, “Mapping” grade GPS units made by companies like Trimble and Leica, which cost more -- priced between $500 and $6,000, have more accurate clocks which permits more precise differential correction and therefore better accuracy.
Some Recretional grade GPS units allow you to average a number of positions into a single Waypoint. The cheapest Garmins (eTrex) don't allow this but mid-range and higher end GPS units do.
In some cases you can test the accuracy of your GPS by collecting a cloud of points and examining the distribution. You must have a way of streaming individual point measurements to a collection unit.
B. Data logging
Recreational GPS units can be used for logging geographical data. From the perspective of Geographical Information Systems (GIS) features are mapped using three kinds of geometry: Points, Lines, and Polygons. GIS Data loggers and mobile GIS units explicitly use these three geometry types for mapping features in the field.
Recreational units will often not have Polygon (Area) geometry types. The best workaround for mapping Areas is therefore to use Line geometry and try to “close” the shape so that you can more easily convert the Line to Polygon on a GIS later on.
As an example, I will describe mapping an archaeological site boundary using a Garmin GPS.
1. Evaluate the site and determine the site boundary. Perhaps use pin-flags to mark the edge.
2. In the Garmin GPS start a Line feature known as a “Track”. You may need to mark the starting point of your Line with two flags or perhaps a backpack so you can return to your starting point.
3. Walk slowly around the edge of your feature and watch how the Track accumulates points.
4. Return to your starting point and close the Track. Some Garmin units have a Track: Calculate Area function that will report the enclosed area immediately.
5. Rather than type in a long name for the feature, which is time consuming, considering just giving these features ID#s that you also use in your field notes. If you are systematic in your ID# system, you can use these numbers as the unique ID#s in a GIS database in the future. Also, you can link this ID# to the JPEG filenames of digital photos of that feature in your field notes. My ID#s were based on the year of fieldwork so that I reuse numbers after each year. For example, the fifth feature I mapped from 2003 fieldwork was numbered '03-0005'.
Comments:
- In a GIS system you may be able to “snap” such lines into polygons or to automate closing of these Lines into polygons by using a particular tolerance for the snapping the beginning and end points together.
- Don’t bother mapping Area features smaller than 2-3 meters in diameter (with WAAS) or 5-10m diameter (without WAAS) since the error of each of your positions makes such small areas meaningless. Use a “Waypoint” instead to map such features.
- Point features (aka Waypoints) make more sense when you’re trying to map the location of individual artifact types of interest. These convert into Point geometry in a GIS.
C. Storage
Your GPS might be pre-loaded with reference map data. If you’re short of memory for your field data you may be able to delete these preloaded reference maps.
You’ll want a Data transfer cable to download the data to a computer. For Garmin a good, free program that exports to ESRI Shapefile format can be found here
DNR Garmin Extension for Arc
http://www.dnr.state.mn.us/mis/gis/tools/arcview/extensions/DNRGarmin/DN...
Also, a plug in for ArcGIS 9 and Arcview 3 can be found there.
Make sure you're using an appropriate Coordinate System and Datum. See my post in this Forum Topic on Coordinate Systems for more information.
For greatest versatility and export potential use WGS84 datum and Decimal Degree format for stored data. From that system you can convert to any other coordinate system.
D. Mobile GIS
Recreational GPS units provide a low cost way of recording spatial location relatively easily. In the past few years a class of GIS software known as “mobile GIS” has emerged that bridges the gap for field research between a recreational-grade GPS and a full-fledged GIS on a tablet PC.
Consider using a mobile GIS instead of the recreational GPS approach described here if the following features are important
1. view of satellite imagery and other custom data (existing layers) during your fieldwork
2. query of existing data layers based on spatial location while in the field
3. random location generation (sampling units) based on field data like site boundaries.
4. checkout and checkin of vector data layers from a larger GIS on a regular basis for fieldwork, and work on multiple field units.
I’ve written a general geography article about Mobile GIS using Arcpad 6 here
"Flexibility by Design: How mobile GIS meets the needs of archaeological survey" Cartography and Geographical Information Science 31.3 (2004): 137-151.
And an archaeology focused article here
"Interfaces: Mobile GIS in archaeological survey" The SAA Archaeological Record 4.3 (2004): 17-22.
Also, a slideshow presentation discussing Mobile GIS in archaeology from 2004 can be viewed here