Adding Ground Control Points (GCPs)
A ground control point (GCP) is a physical location on the ground that has been accurately surveyed by a qualified GPS surveyor.
This involves sending a ground crew to the site. At the site, the ground crew constructs a physical ground control target (GCT), and then collects GPS data at each GCT.
In general, the vertical accuracy of mapping and surveying projects increases with the density of GCPs. Sites such as active mines and oil sands projects normally have a high density of existing GCPs from previous surveys. However, wilderness sites that are of interest for exploration, PEAs or feasibility studies, sometimes have few or no GCPs.
For some sites, adding GCPs may be difficult. Some reasons include:
- Conflict zones: It can be difficult or unsafe to access certain regions where security is not guaranteed.
- Weather windows: In regions such as the Arctic, short weather windows limit access.
- Wilderness access: When targets are far from urban centers, accessing them may be logistically challenging.
Procedure to Add GCPs
To add GCPs in a target area, follow these steps:
Step 1: Determine the required number and location of targets
To determine the number and location of ground targets to add, it is important to consider the size and type of terrain. In general, vertical accuracy increases with the density of GCPs.
Step 2: Choose appropriate location
The site for the ground control target should be clear and on level ground. The target must be 20 m away from buildings, trees, or abrupt changes of terrain elevation (e.g. cliff edge).
Step 3: Establish a ground control target (GCT)
There are several acceptable ways to establish a ground control target. The target must have these dimensions:
Step 4: Use pre-existing project survey benchmarks (if possible)
If an existing project survey benchmark was previously used, all new GCTs must be surveyed relative to that benchmark. The accuracy should be better than 10 cm in x, y, & z.
Step 5: Record GPS data
Follow the guidelines in the next section. See also Information to Supply and Sample: GPS data.
Guidelines for recording GPS data
Project survey benchmarks
GCTs must be surveyed relative to a benchmark. If there is no pre-existing survey benchmark on the project, one of the GCTs may be used as the benchmark. It will be for all future surveying and mapping on the project. We recommend the following guidelines:
- International Association of Oil & Gas: http://www.ogp.org.uk/pubs/373-19.pdf
- Natural Resources Canada: http://www.geod.nrcan.gc.ca/userguide/pdf/howtouse.pdf
Horizontal location and elevation of each GCT
For each GCT, the survey must meet the following requirements:
- Accurate to at least 10 cm relative to the benchmark
- Elevations must represent the mean elevation of the ground within 3 m of the centre of the GCT
Survey time for each GCT
Each GCT must be surveying for a minimum time to achieve 10 cm accuracy.
Total Survey Time = 30 min + (1 min per *km)
*Distance (in km) between the GCT and benchmark.
Information to Supply
If you want GCPs incorporated into your map or survey, supply the following information:
|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|
Sample: GPS Data
When submitting GPS data, please supply a spreadsheet with the following fields:
|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|
NOTE: Provide one file for each occupation of each point. Both RINEX and Compact RINEX are acceptable.
Examples of ground control targets
There are a few acceptable ways to construct a ground control target.
Example 1: White boards crossed on the ground
Figure 1a: This GCT is effective because it was created on even terrain, away from buildings and trees. The white boards create good contrast against the colour of the ground, and the differential GPS unit is in the photo.
Figure 1b: The GCT is clearly visible in the 50 cm resolution satellite photo.
Example 2: White tarp anchored by dirt
To improve this GCT:
- In the photo, show the GPS equipment on the target.
- Use rocks to hold down the tarp. Dirt could easily blow off from strong winds.
Figure 2a: This GCT is effective because it was created on even terrain, away from buildings and trees. The white tarp creates good contrast against the colour of the ground.
Figure 2b: The GCT is clearly visible in the 50 cm resolution satellite photo.
Example 3: White stones
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. These painted rocks will not blow away or be easily destroyed.
Figure 3b: The GCT is clearly visible on the 50 cm greyscale satellite photo.
Example 4: White rocks with wide arms
To improve this GCT:
- In the photo, show the GPS antenna on the target.
- Construct the target with arms that are narrower and longer to make a clearer cross on the satellite image.
Figure 4a: This GCT is effective because it is away from buildings and trees. The white rocks contrast well against the ground colour. The GCT is made from solid, painted rocks that will not blow away or be easily destroyed.
Figure 4b: The target is visible on the 50 cm resolution satellite image. However, the center of the target is difficult to locate within 25 cm.
Example 5: Orange material
To improve this GCT:
- Use a material that contrasts better against the ground.
- Construct the GCT with longer arms with parallel edges.