DEM accuracy

[Emma]

Hi everyone,

I’m using Metashape to create DEMs of hillslope gullies to map erosion processes, so I need to be able to measure the differences in soil height and topography changes down to 1cm accuracy. My plot sizes are 10m x 2m, and I’ve been using coded targets as GCPs set at exactly 1m apart to create scale bars for georeferencing the model.

The problem is when I create a DEM from the 3D model, and use the measuring tool, the accuracy seems to be way off i.e. a soil mound that I know to be only 10cm or so in height in the field shows up as much bigger in the DEM. The elevations are also not turning out very accurate either, with the DEM showing one side of the plot to be 3m higher than the other side, when in the field it was only 50cm higher, so the DEM appears skewed, or the higher side of the gully is actually showing as lower. I'm also confused about the X,Y,Z values that Metashape assigns my model based on the scale bars I've created. Why would the DEM be from say -3.4 to -0.9m - I don't get where these values are coming from. Maybe I’m missing a crucial step when georeferencing the model using the scale bars, but I haven’t been able to figure out what this may be. I can’t use a GPS enabled camera for georeferencing as the resolution of the camera locations won’t be high enough.

I’m also confused by the different options for projection plane when creating the DEM i.e. I understand why measurements within the DEMs created in different projection planes are different, but I’m not sure which option is best to use.

Any help is much appreciated!

Thanks!

[Eichhorn18]

Hi there,

I recently had a similar problem getting my local coordinate system to make sense. I'm not doing exactly the same thing as you since I don't need to georeference my model to a non-local geodedic system (eg. WGS84). I am only intersted in the local scale of my model--however, I made my scale work by using markers, not scale bars. After photo alignment, filtering, optimization of the camera etc., and dense cloud construction, I then place markers in photos and type in the known coordinates. You have to then locate the markers on all photos (or allow metashape to autolocate them with some adjustment by you). You would then make sure the check mark box beside the marker in the references table is turned on, and then you must click the refresh button at the top of the references table.

Whether you type in the coordinates of your markers in local coordinates (i.e. the distance in metres between the markers) or input GPS coordiantes likely depends on the accuracy of your measuring system. As you said you do not have the ability to use a GPS enabled camera due to the accuracy, I assume you have some other way of knowing where your survey plot is located within the world?

I'm not sure how you would translate it from a local coordinate system to a geodedic system, but I think it would be a similar process of assigning some markers in your scene with GPS coordinates. If you know the location of the centre of your survey from some more accurate method (total station, better GPS device) then  you might have to manually determine the coordinates of your markers in your scene. But since you say you know these locations to exactly 1 metre, you should be able to figure out their UTM coordinates provided you have the location of your survey plot (and maybe a compass).

[Emma]

Hi again!

I'm doing the same as you in that at this stage I'm also only interested in the local scale of my model. How did you find the co-ordinates of your markers? Did you measure them in situ?


I'm thinking that the next stage, when I want a geodedic coordinate reference to my model (in WGS84), I'll do what you've suggested in that I place a marker somewhere in my plot and get as accurate a GPS location for that marker as possible.

[Eichhorn18]

Hi again!

I'm doing the same as you in that at this stage I'm also only interested in the local scale of my model. How did you find the co-ordinates of your markers? Did you measure them in situ?


I'm thinking that the next stage, when I want a geodedic coordinate reference to my model (in WGS84), I'll do what you've suggested in that I place a marker somewhere in my plot and get as accurate a GPS location for that marker as possible.

So I'm doing much smaller scale objects. My scale markers were 3D printed in a grid and so I knew the design coordinates of my markers.

That being said, I have experimented with placing a geometric object of known size and dimensions in my photo capture area, with unknown marker locations. I then scale the reconstructed point cloud based on the know size, side lengths of the geometric object, to get the scale right. Then I can pick out the coordinates of the markers in another program by looking manually at each marker point in 3D space. There is likely systematic error involved in this and I cannot say what the accuracy of locating markers this way is. It was only possible owning to the small size of the area I'm photographing (300 mm2), and so likely is not an option for you.

I think your best option is measuring your marker placements insitu. You could simply set up a grid with wooden stakes and string and measure out some rectangles around your gully at different elevations, similar to how archeological digs are mapped out. You could then make the lowest corner the origin, and measure out with a tape the locations of the markers which could be placed beneath the string. It would be a bit of work, but in the abscence of a total station with GPS to the cm or better, you dont really have another option.