I did my photogrammetic work in both Florida, and at another firm I worked at in Tennessee.
The relief is very different in the ridge-and-valley country of Appalachia to that in flat Florida and provided very different challenges to the stereo plotter operator. In TN, you had to carefully adjust the equipment so that the high and low points of the photogrammetric model fell entirely within the instrumental vertical constraints of the instrument. In Florida, you could afford to be sloppy, since the potential delta-Z was well within the high-low limits of the equipment and the photo geometry.

I do know the Z value was deliberately exaggerated by the optics so that the photogrammetrist was able to make accurate measurements of very small elevation differences. For example, when a contour line crossed a road built on a grade, it was always convex down-slope,to reflect the fact that the
crown of the road was higher than the drains along either side. If the road had a curb, the contour line would disappear as it traveled along the curb until it hit the appropriate elevation, whereupon it would reappear and continue across the road. If a river or stream crossed a model, the water level was measurably higher upstream, even in flat FL. We often used lakes, which were of constant elevation throughout, to help level models accurately when our ground survey was off or lacking in sufficient elevation data.

I mention this to give you some idea how accurate the photogrammetric technique can be.
In Florida, particularly, it was essential to get the drainage right to protect future development from flooding.

Thanks for the trip down memory lane. I have great respect for this sort of “blue-collar” work. It financed my graduate school, and convinced me that you didn’t have to be a college weenie to do work that required intelligence and dedication. My colleagues, pilots, photographers, photo lab techs, and draftsmen, exhibited great craftsmanship and integrity. I was very proud to be accepted by them and to work alongside them.

Still it makes me wonder what the difference in angle of repose is for similar materials on Earth vs Mars. Logic dictates that it has to be steeper due to less effect of gravity…Googling around I found this interesting post:

http://throughthesandglass.typepad.com/through_the_sandglass/2012/05/reposing-differently-on-mars.html

not so simple as I initially thought, but a nice rabbit hole to travel for a while.

Elevations were determined by means of a dot of light projected onto the 3D model which could be lowered til it just touched the ground as seen through a binocular microscope, or on a projected screen using either red/blue glasses or cross-polarized lenses, one over each eye, one for each projected photo image. You could lower that dot and read the elevation directly of any point in the model, or you could preset it to a particular elevation and slide the dot until it hit the ground, so you could draw a contour line. A pantograph would transfer your hand motions to a pen that would draw on the map manuscript.

The simplest equipment I used, the Kelsh Plotter,

https://upload.wikimedia.org/wikipedia/en/4/49/Kelsh.jpg

could produce an accuracy of better than a foot from photography flown 3000′ above ground. The standard we guaranteed was every contour was accurate to 1/2 a 2′ contour interval, and every spot elevation was accurate to a foot at that altitude. Accuracy in x,y was 90% of all features were within 1/40″ on the map, regardless of altitude flown. At higher altitudes, the elevation accuracy dropped off proportionally.

Other, more elaborate plotters, were capable of much higher accuracy, although I don’t remember the exact figures. Our accuracy was good enough that we did a lot of contour stockpile inventories and stripmine excavation maps for the phosphate and coal mining industry. Volumes had to be accurate to 1% to satisfy the bean-counters. Those guys were always moving piles of dirt around and they were keen and knowing the amount of material they dug up and quarried out.

After I got out of photogrammetry and into remote sensing and GIS I lost track of what the industry was doing, other than in the most general terms. I knew photogrammetry was now relying on scanning devices and LIDAR elevation data (usually from Government sources) and that it was all done on computers any more. I have no clue how the Martian data was processed, but my guess is there is a radar altimeter on one of those satellites orbiting the planet, and they are deriving their digital imagery from scanners on board. Whether they use parallax to generate their models I do not know.

In the late 1970s I also used the top of the line Wild A10 plotter, probably the most advanced analog machine ever built. A million $ machine, it was placed in a climate-controlled, positive pressure room and corrected for earth curvature.

http://www.wild-heerbrugg.com/images/SNAGHTML15f7b5d.PNG

The two hand cranks operated the pantograph pen in x and y, the right foot pedal raised and lowered the pen, and the left foot spun a weighted flywheel that raised and lowered the dot in the z direction. I could go assholes and elbows all day on this dude, sort of like a rock and roll drummer.

When I’ve more time.

I’ve only viewed the video once and was enthralled by it. Some of the landforms are unfamiliar to me. I was struck by the beautiful talus cones with possible water streaks, the patterned ground of permafrost, and the weird sublimation forms of the ice cap. If you’ve any specific forms that caught your eye, let me know with a screen shot or a time stamp and I’ll do my best to speculate what we are seeing.

And damn those sand dunes are beautiful!

I can recognize a few features, Barchan dunes, dendritic drainage, alluvial fans, but most of that geomorphology looks totally alien. Can you give us a quick tour?

In aerial photogrammetry, the two photographs in a pair overlap by 60%, and the area in the overlap zone is visible in 3D. However, this camera geometry exaggerates the vertical dimension substantially. This makes it ideal for elevation measurements and contour mapping, but the visual appearance of the ground is not very realistic. Trees and buildings appear to pop out at you, much more so than if you were in an aircraft looking out the window.

This used to be done by the plane flying in a straight line over the area to be mapped, the camera exposures timed so that the individual negatives overlapped by the required 60%. Repeated flight lines “sidelapped” so that every spot in the mapped terrain appeared in at least two, and sometimes three frames.

Nowadays, I suppose they use digital line scanners and DTEDs (Digital TErrain Datasets) acquired by LIDAR to provide the Z-component. It used to take two years of training to be able to see in stereo well enough to do the work without supervision. Like everything else I once was really good at, the skills required to do the job have now become obsolete.

Getting old sucks.

Still, the illusion of stereoscopic vision can be created by perspective and foreshortening, as it can be experienced by flying a light plane through mountain valleys or cloudscapes. I hate to be nitpicky, but I spent five very long years of my life doing this professionally as an air photo mapper, and it is such a treat to find someone who actually comprehends the distinction.