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Ground control points for Orthorectification and DEM

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#1
meetlens

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Hi,
I am about to establish Ground control pillars (GCP) for orthorectification.There are rules about how to establish the GCP.They include Horizontal accuracy of 75cm(Rmse), Vertical accuracy of 125cm(Rmse).Also how do I distribute the GCP so as to achieve the best accuracy.How do I estimate the accuracy for both vertical and horizontal. Last but not the least, how do I create a Digital Elevation Model (DEM) for the said area.Please help me because I need it urgently and I am new to this.
Thank you.

#2
frax

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Hi, I deleted your duplicate posts in another area of the forum.

Regarding your question - this are quite specific photogrammetry/surveying questions - I wouldn't have too high hopes in getting good responses...

But for the GCPs, I guess they should be evenly distributed over the surveyed area. Using a GPS you get coordinates, and the GPS should give you the error when you average the points. I guess you would look at features that are easy to locate in a satellite/aerial view.

How to create a detailed DEM for the area - if you can't use a DEM that has already been created (global dataset, national survey etc) you would have to use survey technologies, includinding ground/aerial lidar/radar surveys...
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#3
Earthscientist

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Hi,
I am about to establish Ground control pillars (GCP) for orthorectification.There are rules about how to establish the GCP.They include Horizontal accuracy of 75cm(Rmse), Vertical accuracy of 125cm(Rmse).Also how do I distribute the GCP so as to achieve the best accuracy.How do I estimate the accuracy for both vertical and horizontal. Last but not the least, how do I create a Digital Elevation Model (DEM) for the said area.Please help me because I need it urgently and I am new to this.
Thank you.


Orthorectification implies that you are planning on geometrically correcting remote sensing data for the effects of parallax caused by topographic elevation over the region of interest.

Assuming this is the case, the accuracy you can obtain will depend on the ground imaging resolution of the sensor that collects the data and how accurate its height above the earth surface, its orientation relative to the earths surface and velocity, are known.

As a general rule of thumb, a level of vertical horizontal accuracy that can be obtained with GCPs, for which the horizontals and vertical position of which is accurately known, assuming accurate sensor information, is 0.5 x the pixel dimensions of the RS dataset. So for a 15m resolution RS dataset, you aim for horizontal and vertical resolution of 7.5m. Howewver, this is the ideal and so might not be obtained everywhere or in all cases.

GCPs are usually chosen to provide a uniform coverage across the region spanned by the RS data set and to also span the range of the relief that is present across the region. You should also pick extra GCPs that can be used to provide a check on the horizontal and vertical accuracy of the othorectified results. Having too many GCP scan be as bad as having too few. There comes a point where there is no benefit from adding ever more GCPs . GCPs should be chosen to be at points which are identifiable in the RS data, i.e., there is some variation in the pixel value that allows a GCP location to be identified.

#4
meetlens

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Thank you ,Newbie and Hall of fame.




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