Marking Control Points (GCPs & CHPs) in the Uploader

Now that you’ve captured your data, it’s time for processing! Marking control points helps to increase the accuracy of your data.

Before marking control points in your images, properly format your control points file and upload your control points. You’ll want to be sure that the coordinate system of your control points file matches the coordinate system of your outputs.

Note: There are two types of control points: ground control points and checkpoints. Checkpoints are currently only used for verification purposes if the user reports an issue with the accuracy of their dataset, so you will not automatically receive an accuracy report if you choose to upload control points with a dataset. 

After uploading your control points, follow these steps to manually mark your control points:

1. Leave the Tag ID field blank for each GCP you wish to mark. Click Mark GCP or Mark CHP on the first control point you wish to mark.


2. The app will display a set of images that most closely correspond with the control point’s coordinates.

3. Click on an image to get an enlarged view for locating the control point.

4. Here are a few things you can do when you’re in the enlarged view of an image to identify the control point’s location:

     a. Zoom in and out of the enlarged image by using your mouse scroll wheel.

     b. Click and hold the image to drag your view across different points of


     c. Reset the default zoom view

     d. See the number of marked photos of the control point of interest

     e. Browse between photos

     f. Navigate back to the gallery of photos

5. Once you’ve located the control point’s location, place your cursor over the control point and click to mark it.

6. After you place the control point mark on the photo, you can:

     a. Zoom in on the mark with one click

     b. Remove the mark

7. After zooming in on the mark, you can:

     a. Re-click on the photo to adjust the mark location to the center of the control point. (You should always place your marks in the center of control points for the best accuracy).

     b. After the mark is placed in the center, you can continue to the next photo until the control point has been marked in at least 5 photos.

NOTE: Although the minimum number of photos marked is 5, we strongly recommend marking as many photos as possible per each GCP to get the most accurate results.

8. Repeat the marking process so that your control point is marked in at least 5 separate images. Once you’ve done so, you should see a screen similar to the following screenshot. Click the Complete button to move on to the next control point.

9. When you have marked all of your control points:

     a. Make sure that all the non-automated control points have been marked

     b. Confirm control points step is complete

     c. Validate that the correct number of control points has been added to the uploader

     d. When ready, click the green NEXT button to go to the next step

Note: You are required to upload or mark at least 4 GCPs. There is no minimum or a maximum number of checkpoints required for upload.


Why aren't my GCPs being displayed in the photos?

The Skycatch GCP marking tool will display photos based on proximity to the coordinates entered for each of your GCPs. Some common reasons you may not see your GCP in any of the photos shown could be:
1. The GCP is not present on the site
2. Incorrect coordinates were entered for the GCP
3. The coordinates for your GCP were reversed for X/Y or Lat/Lng
4. GCP coordinates were entered in a different coordinate system than what was set on the site (in Step 2 of upload)

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