GPS Measurements and Errors

In this exercise, we recorded latitude and longitude of Ground Control Points in the UCLA campus. We used a GPS equipment to record these measurements. These measurements were later consolidated in one spreadsheet, and were used to associate coordinates to an orthophoto for the UCLA campus. The satellite signal received by GPS devices should be error free, yet there are multiple sources of errors that as GIS users we should be aware of while carrying out our analysis. The georeferenced orthophoto overlaid on basemap and Ground Control Locations is shown in maps on next pages.

Sometime the GPS device clock could have inaccurate clocks. Clocks are essential to calculate times required for signals to travel. Inaccurate time recording can introduce errors in recording locations. Atmospheric disturbances can also result in signal delay. Satellite signals travel through the ionosphere and troposphere layers of atmosphere before they reach the receiver in GPS device. If these layers are dense, they slow down them down. Electromagnetic radiations wavelengths are inversely proportional to frequency. The errors depend upon the frequency. Though modern GPS devices use procedures that try to account for this error, yet they may not be foolproof. In addition, GPS signal from satellites can be reflected by local sources. These could be local buildings, foliage, or certain types of hats or caps that may cause deflection of the path of signal. This can cause delay in the receipt of signal by the device. Sometime there could be error in satellite orbital position relative to its actual position. The visibility of satellites can also contribute to errors. Greater the number of satellites a GPS receiver can get signal from, greater is the accuracy. Anything that comes in the line of sight of GPS device, such as buildings, rocks and mountains, dense foliage, electronic interference, can introduce errors in positions. Satellite position can also cause errors. For signals to arrive without distortion, orbiting satellites need to be at wide angles so their signals do not interfere. Signal interference can distort signals.

Any interference in signal received is not desirable. This introduces error in recorded measurements of locations. Whenever possible, highest quality of GPS devices should be used because error prone recording of latitude and longitude would introduce errors in georeferencing the orthophoto, or other GIS layers that we may use for spatial analysis. Many types of GIS processing creates additional geospatial data. If the source of data contains uncertainty, the derived data will also have errors, and we must qualify this data with uncertainty as well. This is an important piece of information that is usually missing from the description of data supplied by agencies. In practice, there may not be a way to eliminate 100% of errors in measurements of GPS locations. But we must be aware of the sources of error that the GIS data may be associated with. This will allow us to be more careful in the interpretation of data when they are used in practical GIS applications.