Improving data product accuracy using GCPs during the image capture process.
Accuracy of maps and other data products depends on the accuracy of the geotags on the images captured during a mapping flight and on the ability of the mapping engine to position the map accurately on the earth's surface. A common way to improve the accuracy of maps and other data products is to place ground control points (GCPs) around the area of interest, to be photographed during the image capture process. GCPs have known GPS positions, which provide a "hook" the processing engine can use to more accurately position the final data product.
Measure Ground Control (MGC) supports auto detection of GCPs via its mapping product workflow integration with Pix4D. The combination of machine learning and computer vision pinpoints the exact center of a GCP almost instantly with pixel-level accuracy. AutoGCPs work with three types of targets: square, diagonal, and Aeropoints. Points should have a clearly defined center, and be at least 2 ft x 2 ft
(60 cm x 60 cm) in size.
Example Auto Detected Ground Control Point Patterns
For most accurate results, pilots should:
- Operate drones equipped with Real Time Kinematics (RTK) systems that connect to a base station and use a data corrections service (e.g. NTRIP/CORS). You can also correct data post flight with PPK (Post Processing Kinematics).
- When choosing GCPs be aware that some have built-in positioning systems and others require the use of additional survey equipment to accurately geolocate. A high accuracy GPS device should be used to capture coordinates and elevation for each GCP.
- If you don't have a separate GPS device you can use your drone to collect the GCP coordinates. Take a picture sitting on top of each GCP and later extract the coordinates and elevation from each GCP image's EXIF data. The backup method described, especially with elevation is much less precise, but has proven to improve model results when compared to not using GCPs. Be careful to avoid damage to your sensor when sitting atop a flexible GCP in uneven terrain.
To use GCPs in making new data products and maps:
- Note: GCPs are only available for Pro/Enterprise accounts. Please upgrade your individual account to Pro or contact email@example.com if you are interested in an Enterprise trial in order to use GCPs.
- Ensure that the imagery captured is nadir (ground facing) imagery. Oblique imagery is not compatible with GCPs.
- Place GCPs and record their coordinates (decimal latitude/longitude) within the operational area prior to conducting a flight. Use 5-10 GCPs for each site.
- Be sure to evenly distribute the GCPs throughout the site (do not cluster them in one part). Do not place GCPs on the corners or edges of the mapping area as they will only be visible in a small selection of photos and will be difficult for Pix4D to use when processing.
- Avoid placing GCPs too close to trees/other overhanging objects to ensure they are not obstructed during image capture.
- GCPs must be placed at a distance of at least 10 m/33 ft from each other.
- Refer to the GCP manufacturer's instructions for additional info on recording the coordinates of the GCP. Some use cases, such as mapping an elongated corridor, would require a modified GCP placement.
Example GCP Placement (Source: Pix4D)
- Record the following information about each GCP:
If you need to convert your horizontal coordinates or MSL/orthometric heights for inputting your GCPs/converting your outputted data, please click this support article.
- Latitude/longitude: Absolute horizontal coordinates (decimal degrees, WGS84) (EPSG: 4326)
- Elevation: Measured relative to MSL using EGM 96 Geoid (feet/meters).
- Horizontal and Vertical accuracy: Margin of error (± feet/meters) as reported by the positioning device.
If subsequent attempts to process maps with GCPs are unsuccessful or model results aren’t improved with GCPs, please contact firstname.lastname@example.org for assistance.
- Flights must be flown in nadir, oblique imagery is not supported. Fly the site as normal, and upload the images to the MGC Web Portal.
- After uploading the site images and selecting a data product, the option to add GCPs will be available in the make a map preview step. Add each GCP by entering the relevant information captured by your GPS device. MGC supports up to 10 points per site; a minimum of 5 is recommended for good results.
- As each GCP is added, it will be displayed in the corresponding location on the map preview.
- Coordinate position values must be specified in decimal degrees of the World Geodetic System 1984 (WGS84) (EPSG: 4326). To properly capture elevation and accuracy parameters, please follow instructions and best practice supplied with the positioning device you’ve used to capture GCPs.
- Submit the processing request as normal.
- Note: Currently, MGC does not have the ability to process Checkpoints, only Ground Control Points. We can assist with processing Checkpoints for Pro and Enterprise Customers.
- Once data products are completed, you will receive a Quality Report via email. To verify that your GCPs have been detected and used, see the “Georeferencing” section of the report.
- If your GCPs were not properly incorporated check the following:
- Do the source images show the GCPs clearly visible in source imagery? Avoid high wind and flying too fast for your camera to function properly and save data between capture commands.
- Are there are several images for each GCP?
- Does the drone or sensor need recalibrated? Does the drone have proper satellite coverage, over 7, while flying?
- Double check the GCP coordinates, elevation and accuracy entries provided from your GPS device.