Trimble Ranger Data Collector      http://trimble.com/ranger.shtml

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There are basically four steps that we agreed upon that we needed to complete our creation of the Ground Control Points:

• Office planning of data collection
• Field collection
• Post-processing
• Field placement of Ground Control Points (white X)

For this project, the timeline for the completion of the ten Ground Control Points was one week. With receiving instructions on a Monday the final Shapefile and all supporting attribute information along with metadata completion was to be delivered Friday to the appropriate agencies that will utilize the GPS locations of the Ground Control Points. Having GPS thousands of features within the City of Folsom as my primary job, the development of steps and foresight of project completion was second nature .

…and a zoomed in version of one proposed Ground Control Point

The next phase was to develop a data dictionary for the attribute information for the Ground Control Points. The only thing that was needed to be added apart from the default settings in collecting GPS data was an ID name for each Ground Control Point. The field name was “GCP #” with the numbering sequence beginning with 2009.02.27-01; the year/month/day/location number. All subsequent field names are automatic within our Ranger handheld such as date, time, coordinates, and address. There was a comments field added if any additional information was necessary during and after the data was collected which is done in an editing session in the attribute table.

Field Location Creation (Benchmark) and GPS Collection

                We set the logging interval with the data collector for one second logging interval. This means for every second that passed while we were creating the GPS feature we were receiving the measurement for that location. Also it was predetermined to collect one sec logging interval for 1,000 seconds. This helps with differential correction of the features later and will give us better accuracy as we can throw out those measurements that fall outside of our accuracy limits. It is very important to preplan all aspects of your field collection before GPS work is done. Many factors come into question on the type of features and whom the end users are to be. Many times accuracy is sacrificed for productivity and in the end it seems that accuracy is always the last thing to come into question. Obviously collecting a GPS point for a detention basin does not necessarily have to have the high accuracy and standard deviation as our Ground Control Point.  But if you have the equipment that is capable of giving sub foot accuracy than a little extra time and planning could produce results within the guidelines and limits of your equipment. For the most part the cost of the many different models of GPS equipment falls on its accuracy capabilities. Our goal for accuracy with every Ground Control Point that we planned for and hoped for was sub foot. The higher (better) the accuracy the less errors in our coordinates and thus leading to a sounder bench mark for our orthorectification and future parcel and centerline data caparisons. Although this project is not to inform or teach the aspects of accuracy in GPS its underlining attention to detail can never go without question. It should be a benchmark within itself for all GPS data collection.

           With the planning of where and what times we need to be in collecting these GPS location we were successful in completing all the locations in one day. Giving the time spent at every location 1000 one second logging interval = 16.7 minutes collecting one feature and set up time roughly 15 mins and drive time we averaged a full day’s work just in collecting data. Keep in mind that satellites are not always to the best location in the sky for data collection. Pathfinder Office and with TerraSync (only) which is loaded on the Trimble Ranger gave us an ideal time for low PDOP and the most satellites. Okay so what is PDOP, well it is Percent Dilution of Position. It is the measure of the geometrical strength of the GPS satellite configuration and the amount of error in your position. PDOP less than 4 gives the best accuracy (under 1 meter) and between 4 and 8 gives acceptable accuracy. Greater than 8, well you should not be collecting data. You can set a PDOP mask which will be the upper limit for PDOP that you wish to collect data. If PDOP goes above the mask GPS data will no longer be collected (until PDOP decreases). The default in the industry is 6. The Ranger that we used, the PDOP mask was set to 3. So with the down times that PDOP was high and we were waiting for more satellites to pop their heads up above the horizon we were very close to a full eight hours of GPS data collection just for ten locations.

Post Processing (Office)

Field placement of Ground Control Points (white X)

The .cor file is now exported as a Shapefile and added to an .mxd and each Ground Control Point location has been labeled with the time it was collected to show in this project to show the sequence at which the features were collected and the pattern. They also were given GCP #’s according to their collection sequence. Example 2009.02.27-01, represents the 08:45:20am point at the N/W corner of the below map.

A photo showing the final Ground Control Point painted on the asphalt…

Photo showing the center benchmark (two inch concrete nail), for the GPS location of the Ground Control Point; this is the exact location with which the GPS unit was placed for this location. All other nine locations are as such.

Analysis