GCP File Format for Data Processing

The Skycatch data processing pipeline accepts GCPs for increased accuracy of data processing.

You may use your own GCP markers or use Skycatch GCP markers. The advantage of using Skycatch GCP markers is that we use computer vision software to automatically detect and mark them in pre-processing. This is significantly faster and more accurate than doing it manually. Download the printable files for Skycatch GCPs here.

No matter what type of GCP marker you use, we've made it as easy as possible for you to submit the GCP coordinates, along with your photos for processing, by creating a simple text file. You or your survey team will be able to create this in only a couple of minutes and use it for every successive flight.

Please note - GCPs should have survey grade coordinates as this will have a high impact on the accuracy of the data processing.

File Format Description

The file must have .gcp as the file extension, contain a header line, and include the following information:

  • Header line: "name,proj,x,y,z,tag" or "name,proj,lat,lng,alt,tag"
  • "name" - the name of the GCP
  • "proj" - the projection of the GCP
  • "y" or "lat" - Northing or Latitude value
  • "x" or "lng" - Easting or Longitude value
  • "z" or "alt" - Height above the ellipsoid (note that we are unable to accept orthometric height at this time, but are working on adding it; so if you have orthometric height you need to convert to height above ellipsoid prior to entering it in the GCP file; you can use NOAA's tool to translate)
  • "tag" - April tag ID number on the Skycatch GCP. Align this with the Skycatch GCP number you printed out. The ID number is located in the lower right hand corner of each Skycatch GCP. If using your own GCPs, just number this value sequentially

GCP processing with local coordinates

You can also submit GCPs in local coordinates instead of a known projection. If using local coordinates, please use "Local" instead of an EPSG code in the Projection column and submit a localization file (TP3 or GC3) with your upload. GCPs in local coordinates can not be used for processing without a localization file.


We've attached an example file to the bottom of this article (named Example.gcp) to make it easy for you to get started. Simply download it and use a basic text editing program such as Sublime Text to modify and save the file again.

Here is an example of the contents of a GCP file with 5 GCPs in EPSG:4326, using lat, lng, and alt:

name,proj,lat,lng,alt,tag point1,EPSG:4326,39.3359081,-120.0105040,110.300,1 point2,EPSG:4326,39.3349717,-120.0109998,110.339,2 point3,EPSG:4326,39.3278188,-120.0081549,64.572,3 point4,EPSG:4326,39.3302233,-120.0061178,54.941,4 point5,EPSG:4326,39.3256711,-120.0027976,61.790,5

If you have more than 5 GCPs just make a new row. Remember to increment the GCP number (the last item in each row)

Save your file with a .gcp file extension

Currently the GCP upload only accepts GCP files with a .gcp file extension. Files with a .txt or .csv file extension will not be accepted and will be unable to be uploaded. Mac users can right click a file folder and select "Rename" to rename the file extension. Windows users can follow the instructions provided here to rename their file extensions.

Disclaimer: While Skycatch may provide recommendations and links to external documentation, they do not constitute an endorsement or an approval of any of the products, services, or opinions of the corporation or organization or individual. Skycatch bears no responsibility for the accuracy, legality or content of the external site or for that of subsequent links. Please contact the external site for answers to questions regarding its content.

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