Two types of corrections are applied to AeroPoints when processed using the Known Point Method.
The first is an external correction, where one AeroPoint corrects its position to some fixed position on the earth—the known AeroPoint.
The second step is an internal correction when the other AeroPoints correct their positions relative to the known AeroPoint.
| ℹ️ Note: Local Coordinate Systems and Local Site Survey Benchmark processing options are only supported when using AeroPoints with Propeller. If you would like more information, please contact us. |
Steps
| 💡 Tip: For the most accurate information, you should not take a measurement when AeroPoints are capturing data or stand close to your known point with a rover during your survey. This can potentially cause GPS interference, resulting in inaccurate known point readings. |
- Place the center of the known AeroPoint over a survey mark or another point on the ground with known coordinates. These coordinates should be in a global projected or geographic coordinate reference system, in the form of easting, northing, elevation or latitude, longitude, elevation. Alternatively, survey the top of the known AeroPoint with a rover to determine its coordinates.
| 💡 Tip: You can spray-paint the edges of the AeroPoint stencil to make sure you can center the GCP. |
If you have sent Propeller a local grid calibration file for a Site you have already created in the Viewer, you may use local grid coordinates instead of global coordinates.
- Leave the known AeroPoint in place for the entire survey duration. Turn it on before any other AeroPoints or flights start, and turn it off only after all flights are complete and all other AeroPoints have been turned off. The known point must fully overlap every flight and every AeroPoint capture in the timeline - if it doesn't, your survey will fail processing. See Known point coverage below for examples.
- After uploading data from your AeroPoints, click the PROCESS NOW button for your survey in the AeroPoints portal.
Known point coverage
The known point must be recording data for the full duration of your survey. Its capture window must start before your first flight and end after your last AeroPoint has been turned off.
| ℹ️ Note: If your known point doesn't cover the entire survey, your survey will fail processing. Even a short gap - turned on late, turned off early, or not running during any part of a flight or another AeroPoint's capture - is enough to block it. |
You can check coverage in the AeroPoints portal data timeline before you upload.
What an incorrect timeline looks like
In the example below, Point 1 is the known point. It stopped recording early in the day. Flight 3 and Point 2 captured data hours later, outside the known point's recording window.
This survey will fail because the known point doesn't cover all flights and AeroPoints in the timeline.
What a correct timeline looks like
Here, Point 1 (the known point) runs from before the first flight to after the last AeroPoint was turned off. Every flight and every other AeroPoint falls within the known point's capture window.
Before you turn off your known point, confirm:
- All flights for the survey are complete
- All other AeroPoints have been turned off
Local Grid
You can use the Local Site Survey Benchmark correction method if you have provided Propeller with a local grid calibration file for your site. This can be a .DC or .CAL file (which can be used to generate a JXL in Trimble Business Center) or a .JXL file. Otherwise, use the Global Survey Benchmark method.
Select the AeroPoint used as the known location and fill out the coordinates.
Global Survey Benchmark
Select the AeroPoint used as the known location and fill out the information.
1. Select the coordinate system from the Coordinate System dropdown box. Select Projected if you have an Easting and Northing or Geographic if you have a latitude and longitude.
2. Users can input ellipsoid or geoid heights.
3. Select the appropriate units from the Units dropdown box. Select the Site with your local grid calibration set up from the Site dropdown box if you used local grid coordinates. Otherwise, you will need to select the coordinate reference system of your global coordinates. If you have trouble locating the right coordinate reference system, search for its EPSG code at epsg.io and use the code to find it in the list.
4. Select Measured To either Bottom of AeroPoint (ground) or Top of AeroPoint, depending on whether your coordinates refer to the ground beneath the AeroPoint or the top of the AeroPoint. For example, if you placed the AeroPoint over a survey mark, you would choose "ground," and if you surveyed the top of the AeroPoint with a rover, you would choose "top of AeroPoint." Users can provide the ellipsoid height by default if the chosen vertical datum is none.
You can see what was used to process your AeroPoints in the AeroPoints Dashboard.
Accuracy
Global accuracy: If you place the AeroPoint accurately, the best available, 20mm/20mm/50mm (plus the accuracy of the mark itself).
Consistency: If you use the same known mark each time, the best available, 20mm/20mm/50mm.
Internal accuracy: 10mm or less — precise internal reconstruction.
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