[gdal-dev] Help with finding residual error for a given GCP.

[Hays Barrett]

Hello all, 
I want to get the error for each GCP before warping so I can calculate the RMSE before georeferencing. 
With a list of GCPs, is it possible to get the eventual map location of each pixel coordinate to calculate the transformation error for each GCP? 
I have tried to use GCPsToGeoTransform but that gave me a map location that was further from the intended chosen GCP coordinate than the actual post- georeferencing pixel location. 

Here is my GCPsToGeoTransform code: 
##CODE_START 

from osgeo import gdal 
#list of GCPs 
gcpjson={'0': {'row': 5305.626973704321, 'col': 1158.432032601927, 'lat': -107.18221364936802, 'lon': 35.08107313264506}, '1': {'row': 778.3230540174817, 'col': 2317.0737231100225, 'lat': -107.17634116783263, 'lon': 35.09958594602314}, '2': {'row': 3950.8777704175645, 'col': 4880.465723118029, 'lat': -107.163460244414, 'lon': 35.08679518563491}, '3': {'row': 7609.882307742781, 'col': 4407.027704042081, 'lat': -107.16570574683065, 'lon': 35.07164256912238}, '4': {'row': 1822.0904204049455, 'col': 6486.900377637105, 'lat': -107.15529960157271, 'lon': 35.096105471961536}, '5': {'row': 6291.396402113635, 'col': 7216.1033738731185, 'lat': -107.15180240538085, 'lon': 35.077317337370516}} 

#New list to put in gdal.GCP objects into 
gcpList=[] 
#Load GCP objects into gcpList 
for gcp in gcpjson: 
gcpList.append(gdal.GCP(gcpjson[gcp]['lat'],gcpjson[gcp]['lon'],0,gcpjson[gcp]['col'],gcpjson[gcp]['row'])) 

#create GeoTranform from gcp list 
GT = gdal.GCPsToGeoTransform(tuple(gcpList)) 

#Get pixel location from first GCP as a test. 
col=gcpjson['0']['col'] 
row=gcpjson['0']['row'] 
xp = GT[0] + col*GT[1] + row*GT[2] 
yp = GT[3] + col*GT[4] + row*GT[5] 
print("POINT ("+str(xp)+" "+str(yp)+" 0)") 

##CODE_END 

This prints out the WKT 'POINT (-107.18216298 35.0808978834 0)' which is kinda close to -107.18221364936802, 35.08107313264506 but looking at my post warped image the chosen pixel is right on top of the intended coordinate so I am clearly doing something (or a lot) wrong. 



Any help would be great. 
Thanks, 
-Hays 

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