DEM without GCPs and UAV?

[matt07lx]

I'm trying to work out a method to create a DEM to serve as a base map for an archaeological project in a relatively small area (a property surrounding a historic home). Normally, I use a Total Station to shoot in the coordinates for GCPs, but I likely won't have access to one for this project. These coordinates not only help to georeference the resulting orthomosaics and DEMs, but they provide the x,y plane. To get around this, I was wondering if I could do something relatively simple: get a bubble level with some measurements on it. I would attach a couple targets at known distances for scale, then use the bubble levels to ensure that it was level. The resulting scale bar could also serve as the x,y plane for creating orthomosaics.

The second issue is how to get the proper coverage and image overlap without the use of a drone. I'm guessing that a pole would do the trick, since taking photos from my standing height, even in a relatively small project area, would probably not work.

Would this work? Am I missing anything or making any unintentional mistakes?

[Packy_GIS]

Hi - the point 1 should work. This is the way how to establish a local coordinate system. If you can use a handheld GPS with barometric elevation measurement, you may add z Z value as well (if you will use just the elevation based on the "air pressure"). The measurements of the Z coordinate should be homogenous if you measure them in a short period of time (but globaly not accurate - based on the initial error).

Ad 2 - I do 3D modelling of archaeological sites in Nort-East Africa - here I use kite (KAP), tall step ladder or I just shoot the pictures from elevated spots around the site. You have to think about it carefuly while shooting images "earth-bound".

Jan

[matt07lx]

Thank you, Jan, for your response. I found an academic article where they use a similar methodology for creating models without a Total Station or GCPs. They use a builder's square with levels on each arm to ensure a proper horizontal x,y plane. My only concern would be finding a builder's square large enough - I would figure that to properly scale a larger model you would want one at least 1 x 1 meter? There's also the question of how many. Would a single one be enough? Or would multiple scale bars be needed?

Second, I'm a bit concerned about the ground coverage, which is mainly a grassy lawn. Is there sufficient detail and variability for Photoscan to align multiple images of grass? I normally work in the Mediterranean where the terrain is fairly rocky and dry, so this isn't usually an issue. But for this project I'm dealing with grass - nice for digging, but not so good for photogrammetry. Anyone have success with modelling a grassy landscape from a low altitude?

[Thibaud Capra]

Hello matt07lx,

I'm creating accurate 3D models of golf courses using UAVs, which are, well, mostly grass. Clear patterns would help building your model though: can you set up coded targets on your site?

[matt07lx]

Good to know, Thibaud. I could set up targets just for the purposes of alignment, but I wont have a Total Station to include coordinates for them. Are targets a necessity, from your experience, in a grassy situation? Would I need to have enough so that one was visible in each photo?

[Thibaud Capra]

I could be wrong, but since the grass I have in my landscapes have some kind of patterns from the mowing, I'd say matching and aligning photos is easier.
If your grass is a wild, dense one, I believe it's harder. Having distinctive items in your photos would greatly help IMO.
Not necessarily having a target visible in every photo, but at least 3 would be a good start.
Basically, if you have a decent overlap you should be good.

Keep in mind the 3x3 rules of Photogrammetry!