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Author Topic: Ground Control Error Reporting  (Read 6816 times)

jailblazers

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Ground Control Error Reporting
« on: August 06, 2014, 03:31:54 PM »
I've read through several of the forums regarding how errors for ground control points are reported and still don't quite understand how the errors are being calculated.

From what I've read, the total error reported at the bottom of each column is essentially RMSE ("square root from the sum of squares and that's all is divided by the number of GCPs").  Is this correct? 

I've put my GCP error values into an excel table and no matter how I try to calculate total error for each column (x,y, and z), or the error values in the 5th column, I can't get the numbers even close to matching.

Would someone please help explain how PS calculates these values?

Thanks in advance.

Porly

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Re: Ground Control Error Reporting
« Reply #1 on: August 06, 2014, 04:13:13 PM »
Hello,

root ( (sum v^2)/(n))

should work like this.

where
v = estimated - input data (estimated error)
n= number of GCP

the 3D error (5th column) for each point is the spatial pythagoras.
root (dx^2+dy^2+dz^2) = 3D error

BR

Paul


« Last Edit: August 06, 2014, 04:19:07 PM by Porly »

jailblazers

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Re: Ground Control Error Reporting
« Reply #2 on: August 06, 2014, 04:39:18 PM »
Thanks, Porly

Hello,

root ( (sum v^2)/(n))

should work like this.

where
v = estimated - input data (estimated error)
n= number of GCP

the 3D error (5th column) for each point is the spatial pythagoras.
root (dx^2+dy^2+dz^2) = 3D error

BR

Paul


That definitely helps explain it for me (and hopefully others).

However, calculating 3D error using that method is still giving me a slightly different value (0.355 m) than what PS reports (0.339 m).  What does d represent in your 3D error equation, pixel size?

Thanks

Alexey Pasumansky

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Re: Ground Control Error Reporting
« Reply #3 on: August 06, 2014, 04:44:26 PM »
Hello  jailblazers,

For point 1: Error = sqrt(X_error ^ 2 + Y_error ^ 2 + Z_error ^ 2), as it is actually the distance in 3D space between estimated and source location (the latter one is defined by input coordinates)

For total error, as Paul has already said: Total_error = sqrt( sum(point_error ^2) / point_number)

Maybe you can post the exact equation on how you've got 0.355?
« Last Edit: August 06, 2014, 04:46:23 PM by Alexey Pasumansky »
Best regards,
Alexey Pasumansky,
Agisoft LLC

jailblazers

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Re: Ground Control Error Reporting
« Reply #4 on: August 06, 2014, 04:58:51 PM »
Thanks, Alexey

If you look at my spreadsheet X error= -0.08187, Y error= 0.062554, Z error= -0.323538, and 3D error reported by PS = 0.339547.

When I calculate, SqRoot[(-0.08187^2)+(0.062554^2)+(-0.323538^2)] = 0.354834983









Alexey Pasumansky

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Re: Ground Control Error Reporting
« Reply #5 on: August 06, 2014, 05:03:10 PM »
Strange, I got the following:
math.sqrt(0.08187 ** 2 + 0.062554 ** 2 + 0.323538 **2)
0.3395475478633294

Maybe omitting minus signs will help?
« Last Edit: August 06, 2014, 05:15:41 PM by Alexey Pasumansky »
Best regards,
Alexey Pasumansky,
Agisoft LLC

Porly

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Re: Ground Control Error Reporting
« Reply #6 on: August 06, 2014, 05:05:25 PM »
Me too