# Forum

### Author Topic: DEM accuracy vs ORTHOMOSAIC accuracy  (Read 2178 times)

#### matteosin

• Newbie
• Posts: 17
##### DEM accuracy vs ORTHOMOSAIC accuracy
« on: December 03, 2018, 09:15:26 AM »
Hello everyone,

do you guys know why the accuracy of the DEM and the accuracy of the ORTHOMOSAIC (generated from DEM) are different?

I know that the GSD (Ground Sampling Distance) depends on the parameters of the camera (mainly camera resolution) and the flight altitude.
In one of my project I have used a Sony camera UMC-R10C 20.1 Mega Pixel and average flight was set at 200m AGL. With this input the pixel size achieved should be 5cm.

This 5cm pixel size is exactly what I have got for the Orthomosaic but the pixel size of DEM is 20cm.

Anyone can explain?

Thank you

#### BobvdMeij

• Full Member
• Posts: 157
##### Re: DEM accuracy vs ORTHOMOSAIC accuracy
« Reply #1 on: December 03, 2018, 09:15:25 PM »
Just to be sure, are you referring to accuracy or resolution? Looking at your text I reckon the latter.

The maximum allowable/highest resolution of your DEM depends on the Quality Parameter set during construction of the Dense Cloud. When the GSD of inputted images is 5cm the maximum allowable resolution for the DEM is as follows with respect to the quality parameter for the dense cloud step:

Ultra high  = 1 * 5.0 = DEM resolution of 5 cm
High          = 2 * 5.0 = DEM resolution of 10 cm
Medium    = 4 * 5.0 = DEM resolution of 20 cm
Low          = 8 * 5.0 = DEM resolution of 40 cm
Lowest     = 16 * 5.0 = DEM resolution of 80 cm

So in other words you will only be able to achieve the same DEM resolution as your image/orthomosaic GSD if you set this to Ultra High. Considering you only get 20cm I assume you generated your Dense Cloud at medium quality, correct?

#### matteosin

• Newbie
• Posts: 17
##### Re: DEM accuracy vs ORTHOMOSAIC accuracy
« Reply #2 on: December 04, 2018, 03:20:16 PM »
You are right. Exact!!