ArcGIS Pro 3.5.4: What does the red color indicate when an image is georeferenced?

Question

In the first four attached images, the coordinates for the indicated points are marked. These coordinates represent the XY values of the image as it is initially inserted into ArcGIS Pro. These points are then used to georeference the image according to the actual reference points shown on the grid.

After adding four control points, two colors appear: green, which represents the control points used to georeference the image, and red. However, there is no explanation of what the red points represent. How are the coordinates of the red-colored points calculated?

Parcel.jpg

MikeMillerGIS_1-1743013291209.png

RTPL_AU · Answer

Hi @JamalNUMAN 
Two things
1 - the colours could just as well be pink & brown - they do not define good/bad or anything else. 
I think red is source pixel location and green is map coordinate location.

2 - the distance between them is based on the relative error when the georeferencing method cannot line them up 100%. 
Think of a point graph in Excel that you want to fit an equation to. The more complex the equation the better you can match the line to the points, but unless the data is perfect there will always be a difference between the calculated line and actual data points. Same with georeferencing.

JamalNUMAN · Answer

Thank you for the input.
However, what do the red and green colors represent?
It is not obvious how the pixel coordinates and the x–y coordinates are compared to one another to derive an error.

RTPL_AU · Answer

Hi @JamalNUMAN

In the georeferencing process you have two sets of coordinates - as you point out in your write-up.

There are the coordinates of the points in the raster image itself (listed in pixels) and the 'real world' locations these raster points are linked to.

If the raster and grid points do not perfectly align you will end up with what you see - two separate points at each link location. One for the raster location and one for the grid location. The distance between the two is the resultant averaged error based on the method,1st order poly in this case, and the number of points and their overall position/distribution.

In a perfect world the green & red will be on top of each other but with raster pixel sizes, paper scan distortions, mistakes by the original cartographer, etc., all we can do is to get the error as small as necessary to get the job done.