Guidelines for Stereo Satellite Ground Control Targets
1. Stereo satellite ground control target locations
The number and location of requested ground control targets varies with the size and terrain of the specific project.
2. Ground control target local conditions
The ground control targets should be on areas of level ground at least 20m from buildings, trees and abrupt changes of terrain elevation.
3. Ground control target dimensions
4. Existing project survey benchmark
If there is an existing project survey benchmark that has been used to reference surveying of drill hole collars and other engineering features on the project the stereo satellite ground control target must be surveyed relative to this benchmark, with an accuracy of better than 10 cm in x, y, & z.
5. Establishing a project survey benchmark
If there is no survey benchmark on the project one of the ground control targets may be surveyed to become the benchmark for the stereo satellite mapping and for all future surveying on the project. We recommend the following guidelines, derived from the June 2011 “Guidelines for GNSS positioning in the oil and gas industry” of the International Association of Oil & Gas producers (http://www.ogp.org.uk/pubs/373-19.pdf) and the Natural Resources Canada “On-line Precise Point Positioning ‘How to Use’ Document” (http://www.geod.nrcan.gc.ca/userguide/pdf/howtouse.pdf).
6. Ground control target survey guidelines
The horizontal location and elevation of the stereo satellite ground control targets must be surveyed, accurate to at least 10cm, relative to the benchmark. The ground control target elevations must represent the mean elevation of the ground within 3m of the centre of the ground control target.
The ground control targets may be surveyed relative to the survey benchmark with a total station or with differential GPS. For differential GPS surveying we recommend the following guidelines, derived from the June 2011 “Guidelines for GNSS positioning in the oil and gas industry” of the International Association of Oil & Gas producers (http://www.ogp.org.uk/pubs/373-19.pdf).
Each target should be surveyed for at least 30 minutes plus 1 minute per kilometer of distance between the target and benchmark to achieve 10cm accuracy.
The following information, including the raw GPS data in RINEX or Compact RINEX format, should be provided to PhotoSat so that we can confirm the accuracy of the benchmark and ground control targets that are to serve as the reference for the stereo satellite mapping:
|Equipment||Make and model number of the benchmark (base) dual frequency geodetic quality GPS receiver and antenna
Make and model number of the ground control point (rover) dual frequency geodetic quality GPS receiver and antenna
|Photograph||Digital photographs of the GPS antennas taken from 3 different angles while the benchmark and the ground control target is being surveyed|
|Recording time||Benchmark A minimum of 10 hours. Oil & Gas company Surveyors and Geodesists tell us that they usually record the GPS signals at project benchmarks for 24 hours
Targets Simultaneous GPS recording at the survey benchmark and the ground control target for a minimum of 30 minutes plus 1 minute per kilometer of distance between the target and benchmark
|Data format||Raw GPS data for both the base and the rover in RINEX or Compact RINEX format
Provide one RINEX file for each occupation of each point
|Antenna Height||Height of the base GPS antenna reference point (ARP) above the benchmark
Height of the rover GPS ARP above the bare ground surface at the centre of the ground control target
|ARP to APC||The distance between the antenna reference point (ARP) and the antenna phase centre (APC) for both the base and the rover|
In addition to the information above, please provide a spreadsheet containing the following survey coordinate data:
|RINEX filename*||Site ID||Antenna ARP Height||UTM East (WGS84)||UTM North (WGS84)||East (Project projection and datum)||North (Project projection and datum)||Ellipsoidal Elevation (WGS84)||Orthometric Elevation
(height above sea level)
|Ground target description|
*Our GPS processing system requires the RINEX data to be in separate files for every occupation of each point.
Examples of ground control targets and corresponding satellite images
Figure 1a: This target is effective because it was created on even terrain, away from buildings and trees, it contrasts the ground colour, and the differential GPS unit is in the photo.
Figure 1b: The target is clearly visible in the 50cm resolution satellite photo.
Improvements: The accuracy could be improved if the photo included the GPS equipment on the target. The dirt that is weighing down the tarp could blow over and cover the tarp in strong winds. Rocks are recommended when possible.
Figure 2a: This target is effective because it was created on even terrain, away from buildings and trees, and it contrasts the ground colour.
Figure 2b: The target is clearly visible in the 50cm resolution satellite photo.
Clarification: The ground control target survey elevation must be the ground elevation, not the elevation of the top of the monument.
Figure 3a: This target is effective because it was created on even terrain, contrasts the ground colour, and is made from painted rocks that will not blow away or easily be destroyed. The target is clearly visible on the 50cm greyscale satellite image.
Figure 3b: The target is clearly visible on the 50cm greyscale satellite photo.
Improvements: It would improve the confidence in this target if the photo included the GPS antenna on the target. The arms of the target should have been narrower and longer to make a clearer cross on the satellite image.