> Radar clearly misses 90% of the shadows cast because it does not include vegetation. Radar only reflects off the ground, making objects such as trees and buildings invisible.
This doesn’t sound right to me. Certainly radar can, at certain bands, see through foliage (so-called FOPEN). I don’t have familiarity with radar seeing through buildings at the sorts of ranges and coverage rates that you’d want to use for ground mapping.
The article references the Shuttle Radar Topography Mission, which should use C- and/or X-Band radars — both should see returns from foliage and buildings.
Without digging more into it, my thoughts about the reason behind absence of foliage and building shadows in the radar data are potentially: 1) resolution of the radar data is too low (tens of meters or more), 2) maybe post-processing of multiple radar passes with different geometries to get rid of them, 3) steep grazing angles from the radar not generating much shadow to begin with.
SRTM's FAQ shows that the argumentation in the article is backwards.
Did the radar sample the tops of trees or the ground level?
The radar does not "see" through thick vegetation canopies. It probably penetrated a little way into some canopies, but in general it followed near the top of the canopy.
Did the radar signal bounce off treetops, or topography, or some combination of both that will provide separate data sets (geodesists like myself care about topography, whereas scientists more interested in forestry care about the height of the canopy).
Unfortunately, the wavelength used, 5.6 centimeters, didn't penetrate vegetation very well. That means, for moderate-heavy vegetation, we mapped near the canopy top. We did penetrate a little, as some studies comparing our technique with laser altimeters showed, but not to the ground. If the vegetation was sparse, or had no leaves, we might get a return from the ground. The Vegetation Canopy Lidar, scheduled to fly as part of the Earth Observing System, will have this capability, which may provide some interesting data-set comparisons.
Good clarification. My understanding is that SRTM data did not include buildings or foliage (maybe they filtered out all data except the lowest elevation values?) but that's not "radar" in general.
SRTMv3 is 30m/px covering latitudes ±60. The earliest release was 90m/px. There was also release that was 30m within the United States and 90m elsewhere.
> steep grazing angles from the radar not generating much shadow
Mountain shadows are actually a problem! Some releases contain voids where there was no radar return, particularly around the Himalayas. There's a body of publications on void filling the SRTM data, if you're interested.
This doesn’t sound right to me. Certainly radar can, at certain bands, see through foliage (so-called FOPEN). I don’t have familiarity with radar seeing through buildings at the sorts of ranges and coverage rates that you’d want to use for ground mapping.
The article references the Shuttle Radar Topography Mission, which should use C- and/or X-Band radars — both should see returns from foliage and buildings.
Without digging more into it, my thoughts about the reason behind absence of foliage and building shadows in the radar data are potentially: 1) resolution of the radar data is too low (tens of meters or more), 2) maybe post-processing of multiple radar passes with different geometries to get rid of them, 3) steep grazing angles from the radar not generating much shadow to begin with.