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Need advice for imaging a steep ridgeline

Posted by Tom Mahood on July 30, 2014 at 9:26am

I'm interested in imaging a possible archaeological site located along the spine of a fairly steep descending ridgeline and am looking for advice on how to approach the imaging. At the very least I'd like to be able to produce a high resolution mosaic of the site, but ultimately I'd like to have a 3D surface generated pending establishment of ground control points.

The area I'd like to image is roughly a rectangle 200 meters by 500 meters, with the ridgeline centered along the long axis of the rectangle. My problem is that from one end to the other of this ridgeline it descends 140 meters.

I'm not sure if this is best approached by trying to maintain a constant aircraft altitude (fairly easy) or a constant elevation above the ground (much harder).

I'll add that the site is fairly remote and takes some time to get to so I can't just experiment with different approaches easily. I  have both a multicopter and fixed wing available as tested camera platforms, the imaging camera being a Canon SX230 GPS tagged camera.

Anybody done imaging on very rough terrain with advice? Thanks!

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Replies

  • Steve Zeets August 4, 2014 at 12:38pm

    Tom,

    I did this once, however there wasn't as much vertical difference.  But, it was in a small footprint area.  I mapped a sledding hill that was long and narrow with a lot of vertical relief.  Like iskess said, you just have to follow the slope of the land and step-down each flight line as the terrain decreases.

    So long as you have a rough idea of the vertical differences, and it sounds like you do, then you can plan out a flight mission.

    You can use the altitude from the barometric sensor as your elevations and make everything relative to your take-off point elevation.

    Just make sure to figure in plenty of extra overlap so if your estimated elevations are off, then you still have good coverage.

    If you want more specifics or examples of my mission, I can explain in more detail.

    Basically, for each flight line I decreased my altitude so my copter stayed at a more constant AGL.

    -Steve

    • Tom Mahood > Steve Zeets August 7, 2014 at 2:49pm

      Thanks, Steve. I guess I'll plan on an attempt with a longer duration multicopter rather than fixed wing. It's a small enough footprint that it would be hard to match the elevation change with a fixed wing. A copter would give me much better altitude control at the cost of much reduced flight times.

  • Gordon Hamm July 31, 2014 at 9:07am

    Please give me the lat/long position of the ridgeline.

    • Tom Mahood > Gordon Hamm July 31, 2014 at 10:18am

      Gordon, since it involves a possible archaeological site, I'm not comfortable releasing location specifics. But generally speaking the location is on BLM land in NW New Mexico a bit southerly of Bloomington. Beyond that I'd really rather not say.

      • Gordon Hamm > Tom Mahood July 31, 2014 at 5:06pm

        Hi Tom,

        Here is a satellite image of the area around Bloomfield New Mexico.

        Each square of the grid I laid on the image is 10 meters. The resolution is 0.119897 meters/pixel.

        I live in Canada so I would not be interested in the archaeological site. So if the image has enough detail I could give you an image of the required ridgeline.

        Ridge Sample.jpg

        • Tom Mahood > Gordon Hamm July 31, 2014 at 10:20pm

          Hi Gordon....I located your sample image in Google Earth and that appears to be the exact same image at the same resolution. I'm looking for at least order of magnitude resolution better, which is why I'm considering imaging the location myself.  Your sample is centered roughly at 36.662607, -107.971899,and  is not far from my area of interest and pretty much illustrates the difficult terrain I'm dealing with. Deep parallel canyons with steep slopes. Very convoluted and irregular terrain with surface features that don't show up on the best available topo maps.

  • iskess July 30, 2014 at 12:01pm
    I would do your best to follow the contours with each flight line, allowing a large overshoot to provide a reasonable climb/descent gradient. You won't be able to use the MP Survey Tool for this, it is 2D only. You won't be able to maintain a constant AGL, but you will be able to smooth out the terrain a lot. Just be sure the at the apex of the ridge, you can maintain your overlap and sidelap, which means this will be excessive for the rest of the mission.
    • Tom Mahood > iskess July 31, 2014 at 8:50am

      Thanks for the ideas. I had hoped to do something Autonomously but given the ruggedness of the terrain I guess that's just not going to work. The terrain is an area of extremely eroded mud hills with large vertical relief. Were this a relatively flat parcel it would be no problem.

      So It sounds like I need to use a multicopter (too narrow a strip for fixed wing) making curving passes at right angles to the ridge, trying to hold a constant AGL. And each pass should change in altitude as it moves up or down the ridge spine. As I feared, this has the potential to be a real pain in the butt with me screwing it up somehow.

      • iskess > Tom Mahood July 31, 2014 at 11:57am
        I don't see why this can't be don't autonomously with a fixed wing, it will require careful manual flight planning. The AGL doesn't have to be perfectly constant, you are just stepping your flight lines to smooth out the relief as much as practical.
        Post some Google Earth screen shots with the location deidentified so we can get a sense of the shape.
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