The Accuracy of GPS
Global Positioning Systems (GPS) relies mainly on satellites for accuracy. In order for GPS to properly work a GPS receiver has to be able to receive signals from a satellite. The majority of civilian (meaning commercially available) GPS units have an accuracy of 10m-100m mainly due to their reliance on just satellite signals. Another reason for this large range of accuracy is due to the satellite signal coming through the atmosphere. Some higher-end GPS units (i.e. eTrek) receive not only the satellite signal, but Wide Area Augmentation Signal (WAAS) so that they have increased accuracy. The accuracy of these higher-end GPS units is around 3m. The military has even more accurate GPS units which use PPS instead of SPS. The PPS system means that a military GPS unit receives two satellites instead of the one that the SPS system uses. This means that military GPS systems have sub-3m accuracy. Although more technology and signals can increase GPS accuracy the environment that the GPS receiver is in can also impact the reception of the GPS signal. Things such as dense foliage, surrounding builds, and direction of movement can impact the accuracy of GPS causing bias or blunder. Blunder, which is unforeseen errors in GPS data, can also be caused by irregular electricity flow from batteries, user errors, and the like. That being said there are yet further ways to improve accuracy including post processing and Real Time Kinematic (RTK) GPS.
The Accuracy of GPS in the Field
On March 31st we went to Marion Square to experiment with GPS. Initially we had problems getting our clock set to the proper time and the batteries needed to be replaced. These two issues could have resulted in the seeming blunder that was evident in our GPS waypoints and track. The reason that it appears to be blunder is because there is seemingly little to no pattern in the error. If there was a pattern in the error then it would be considered bias. Another reason that it appears we have bias is because our equipment, an eTrek GPS receiver, was not always within its three meter accuracy range.
How to Hyperlink Images and Why Hyperlinking is Important
Hyperlinking an image to a GIS can be useful and can add another level of data to what the GIS is displaying. First, one should, if necessary, separate out the images they wish to hyperlink and rename them so that their image numbers match up with their waypoints. For example, if the initial image file name was IMG00097.jpg and is going to be used for Waypoint 16 the image needs to be renamed to IMG00016.jpg. Before going further one should turn on hyperlinking. This can be done in ArcMap 9.x by going to Properties ? Display ? Support Hyperlinks Using Fields ? clicking “hyperlink” ? and making sure “Documents” is selected. There are two ways to set hyperlinks: explicit and relative path. The explicit method is easier so let’s start with that one. First, open up the attributes table for the waypoints you want to hyperlink. Select “Path” and open the field calculator. Set “Path” equal to the folder path where you are storing your image files you want to hyperlink (note: don’t forget to put the folder path into quotes). Set the “File Name” in the field calculator by typing in the appropriate prefix (i.e. IMG) plus “waypoint number” plus image file format (i.e. .jpg). Now you can easily set the hyperlink in the field calculator to “Path” plus “File Name”. Although this explicit method is easy, it is not the best way. An implicit or relative path is much more flexible because you can move the imagery from computer to computer, hard drive to hard drive. Fist, set the “Path” in the field calculator to “\data\imagery\”. Next, set the “File Name” in the field calculator by typing in the appropriate prefix (i.e. IMG) plus “waypoint number” plus image file format (i.e. .jpg). Now you can set you hyperlink in the field calculator to “Path” plus “File Name”. Then go to File ? Document Properties ? Hyperlink Base: (insert file path here i.e. C: \gps_project\). Now all you have to do is click on the hyperlink tool which looks like a lightning bolt and select one of the waypoints which should display as a blue dot on your GIS.