Collecting Field Data


Here are few things to consider to make field collection of GPS data for your GIS a little smoother.

The GPS unit, additional hardware employed, and data collection software utilized will determine what type of features you can collect. READ MORE >>

When a GPS Unit receives a signal from GPS satellites, it computes a position. The position computations are stored as coordinates. points, lines and polygons are collections of positions. Some GPS units can only collect individual positions, and require additional post processing or editing in the office to create lines or polygons. Most mapping grade GPS units have the capability to output the collected positions into either a point, line or polygon feature. If your GPS unit has the capability of using a data dictionary or a form to collect attributes, these tools can help guide you in determining whether to collect features as points, lines or polygons. See the Project Management & Data Dictionary Design step for more information on attribute collection tools.

Consider field conditions. READ MORE >>

Weather, size of the area to be mapped, accessibility of terrain, and physical obstructions can affect satellite visibility at different locations. For best results, a GPS antenna must have a clear view of the sky from horizon to horizon. It can then receive satellites from any direction. The satellite signals do not penetrate metal surfaces, buildings, tree trunks, or similar objects. The signals weaken when they penetrate the leaves of tree canopy, glass, plastic, and other similar obstructions. Use mission planning to determine the best time to collect data. See configuring GPS Unit & Mission Planning step to learn more about mission planning efforts.

Identify where and how the data is to be collected. READ MORE >>

Decide on a start point and a route that makes the data collection as efficient as possible. Some GPS units have the capability of uploading and displaying existing datasets such as roads, trails or imagery. This can be very helpful in the field because it will show the data that you have collected in relation to the surrounding area, allowing you to plan your next step. If your GPS does not have this capability, it is always a good idea to have a hard copy map of the area with you in the field.

Nesting - When mapping a linear feature such as a road or trail there may be points or waypoints, along the route that you want to map. You can use a nesting function, which allows you collect a point while you are in the process of collecting a line without closing out the line feature.

Offsets - Use offsets at times when you can not come in direct contact with the feature you want to map. The feature may be under heavy canopy or next to a building that blocks most of the sky and causes multipath interference. For point features, collect positions apart from the feature and record the distance and bearing to the feature. For line and polygon features, collect positions apart but parallel to the feature and record which side the feature is from the recorder's direction of travel (left or right) and the distance to the feature. See the Watch out section for more information on offsets.

Perform data management and data documentation in the field. READ MORE >>

Staying up-to-date with data management in the field ensures that the data you collect provides you with the correct answers once it is in the GIS. Using appropriate filenames is an important part of collecting and processing your data. To keep track of datasets, establish a file naming convention for your office. With GPS, you can collect enormous quantities of data quickly. Keeping track of that data can become a problem if you do not properly manage the files. All personnel within a single office should adopt the same conventions and use them consistently. Filenames should include some indication of when the file was created. Take notes about the files you create, note the date, time and purpose for collection.

Collecting attributes about the GPS features creates more robust data. A data dictionary will help you collect important attributes about the feature.  If your GPS unit does not have the capability of using data dictionaries or electronic forms, it is important to take careful notes about the features you collect. A predefined hardcopy questionnaire or form can help guide you in collecting the right information about the features.

Watch outs!

Altering your GPS units configuration settings - After several days of data collecting, some of the GPS unit configuration settings standards worked out in the pre-collection steps may need to be altered or tweaked. Do so on an as needed basis, keeping in mind that all GPS units in the current project should be standardized and documentation of the standard should be updated.

Collecting an Almanac - Before any data collection, be sure you have stored today's almanac on the GPS receiver. This ensures rapid acquisition of signals and more reliable data. This may take 15-20 minutes depending on visible sky and when the receiver was last updated. A good rule of thumb is to always turn your GPS on in the open before your mapping day begins.

Offset Compass bearing and GPS - Offset feature collection of a point requires a bearing from the collector to the feature. Be aware that some compasses read the bearing from magnetic north while other compasses can adjust to true north. When entering your offset bearing into the GPS unit be sure to synchronize the compass's bearing from north (true or magnetic) with the type of bearing the GPS unit is configured to accept. If your GPS does not have an offset option you can always take a position where you are standing, note the bearing and distances of the feature and adjust the coordinates back in office in your GIS software.

Pause It - Some GPS units have a pause button. Use it liberally while collecting line or polygon features. Once you have started a line or polygon feature with a timed logging interval set, any stopping or lingering will cause the unit to collect positions on top of one another. These positions must be deleted manually later, so use the pause button if you stop while collecting a linear or polygon feature. To see an illustration of non-paused output, check this out!!! Graphic credit: GPS Field School Training Manual - Cultural Resources GIS, May 1998

Only map things you can resolve. - Example: If you are using a GPS unit that gives a 4-6 meter horizontal (XY) accuracy, you can not map anything as a polygon that is less than 4-6 meters in width. You would have to capture the object as a point feature.

Reference Features - By collecting point features like anchors or beginning and end points, you can better edit zigzagging line features later. Since points are always more accurate than an instantaneous position along a line (assuming you averaged that point), you can straighten out a line or close a polygon more accurately using the reference point feature as a guide or snapping feature later in the GIS.

Set Limits - Collecting more data increases the database's value. However, keep time and memory limitations in mind. More data and redundant data take longer to collect and longer to process.

Glossary of Terms

Azimuth - Any directional value being measured as an angle that is read in a clockwise direction from a north (O) reference line. When expressed in degrees, its value ranges from 0 to 360. A compass heading is an azimuth. In many GPS receiver documentation, the word bearing has grown to mean the same as an azimuth.

Bearing - A bearing is your direction of travel or the direction between two points. Like an azimuth, a bearing is measured in reference to true or magnetic north, but its value never goes over 360.  The use of the word bearing has changed over the years and now means the same thing as azimuth.

Feature - A physical object or location of an event. A feature can be a point (a tree or a traffic accident), a line (a road or a river), or an area (a forest or a parking lot)

Line - A series of connected points make up a line. It is one-dimensional, having length but no width.

Nesting - When recording a line or area feature, you will often come across point features which you would like to record. You do not have to complete the whole line or area before returning to record the point features; you can just nest the point within the line or area.

Offset - This is used to calculate the position of a precise geographic location that for some reason you can not physically be at.

Point - A point represents a feature for which only one geographic location is needed.

Polygon - An area enclosed by lines. It is two-dimensional; the area enclosed by a polygon has length and width.

Position - The latitude, longitude, an altitude of a point. An estimate of error is often associated with a position.

Track log - Points automatically stored when you are moving. It is the electronic equivalent of laying down a "breadcrumb trail" to mark your path. These points will be shown strung together on the map page of your unit so you can see where you have been.

Waypoint - A coordinate pair of latitude and longitude or northing and easting with an assigned name and number. A waypoint usually represents some geographical point of interest that you would like to navigate to or from. The number of waypoints that can be stored depends on your data collector.

Links (Step-by-Step How To)

Field Note Sheets: General GPS Collection Information: Garmin Information: Trimble GPS units Information:

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