WayPoints - How necessary is it to adujust flight height according to terrain irregularities in photogrammetry and 3D modelling?

[hank970]

So you mean, it is not necessary. I trust that, OK. So, shall we assume that the triangulization we get will not be very different as per the real values.

But, note that the flight height of a satellite is so big that, the difference in the terrain heights covered, would be very negligible compared to their flight height, while I am talking about a drone flight.

The following remarks assume that you want to create a topographic map or something similar. If my assumption is flawed, then disregard the following.

Mean sea level (MSL) serves as a worldwide reference for ground elevations. What you want/need to do with your drone is to create a proxy local reference that is, to the extent possible, a consistent height above MSL. To establish that reference, ascend to a height above a launch point of known elevation that's high enough to safely clear all local terrains. Fly your drone at that proxy altitude while mapping and rely upon it as a fixed reference against which to gauge differences in the elevations of the ground below. The rest is math.

 

[brett8883]

Further to the discussions above, I think I could not explain the core point. Thanks for the valuable feedback above, but I am afraid the real point is missed.

Please do not stick to the figures 20mt-40mt etc given above, these are for examples only, I just wanted to pinpoint the huge AGL variations in the suface, wrt the HomePoint . Suppose, I asked for a flight at 70mt-120mt height or whatever you wish.

My question is this:

The terrain is very irregular and full of hills, valleys, slopes etc. I need to the take the 3D model of this area. If the entire flight height is taken as fixed at X meters above home point, the detail of the subsequent photos taken at some waypoints at different sections of the whole area will be different, because some pictures will be taken at (X+30) meters and some others may be at (X-40) meters.

Will this cause the preplanned GSD to be still realistic?

How can I be sure that, a measurement taken on the screen will be exact? Should I consider such a deviation as negligible?

Bearing this in mind, should I arrange different flight heights for different waypoints at different sections of the terrain or keep it fixed for the entire flight, because this is completely compensated by the camera or the photogrammetry software to be used?

Since you aren’t using RTK or ground control points and using a consumer drone which does not have 3rd party app support for mapping missions, I would say your expectations for accuracy are going to have to be measured.

If matching the waypoint height to the terrain would increase accuracy, I’m not saying it would, any gain by doing that is likely marginal compared to the loss in accuracy you’d get by not doing these other things.

I don’t know this for certain but I suspect the purpose of terrain follow for mapping missions would be to create a constant resolution in the resulting model. Not accuracy. If you wanted more accuracy you’d use GCPs and RTK.