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Re: [TowerTalk] Terrain data for HFTA

Subject: Re: [TowerTalk] Terrain data for HFTA
From: Jim Lux <>
Date: Sun, 21 Aug 2011 16:22:53 -0700
List-post: <">>
On 8/21/11 2:55 PM, Michael Tope wrote:
> Using the Radio Mobile program, I found the SRTM 1 arcsecond elevation
> data to match very well with GPS readings I took in the high desert area
> just north of Los Angeles (to within a few feet typically). The 1
> arcsecond data set has a pixel size of about 30 meters x 30 meters, so
> for any given pixel the reported elevation is the average of the
> elevation over that pixel. For gently sloped terrain this average
> elevation should be a very good approximation. In areas where the
> elevation changes rapidly over the pixel area, there will be more
> disagreement between the elevation reported in the SRTM data and the
> actual elevation. IIRC the requirement on the SRTM instrument was 16
> meter absolute elevation accuracy and 10 meter relative accuracy, but
> the folks I recall talking to who worked on the instrument were elated
> that the performance was typically much better than the minimum
> requirements.

If there's a big discrepancy between SRTM and "on the ground", I'd look 
for mismatches between the reference ellipsoid that SRTM uses and local 
sea level datum.  There can be surprisingly big differences.  If the 
ellipsoid happens to be 100 meters above local sea level, then SRTM data 
for the beach is going to show -100m.

The mean sea level of the earth is not smooth to 100s of meters with 
respect to a perfect ellipsoid.  (e.g. the Gulf stream sits several 
meters above the surrounding ocean, and the ocean surface over deep 
trenches is depressed)

Not only that, but there's both a WGS84 ellipsoid and a WGS84 geoid 
which can differ by 10s of meters in a given location.  SRTM data has 
been published with both references.

These kinds of things are large scale offsets, so they basically make 
all the elevations high or low for 10s of km, and wouldn't have much 
effect on HFTA, which is more about relative elevation.

For places with poor radar reflectivity (water!) with poor SNR, the 
original processed data is known to have spurious peaks/holes on the 
order of tens of meters.

There's also the well known "tree canopy" problem with radar data sets. 
  In dense forest, the surface appears to be the tops of the trees.

W4EF's secret lair in the high desert is located in an ideal location 
for SRTM to work well (aside from being within a few km of some of the 
calibration targets, I'll bet).  It's relatively smooth terrain, has no 
forest (not even a whole lot of trees), and there's no significant body 
of water more than a couple meters across within 100km, except for 
aqueducts and drainage ponds.  It has probably been SAR'ed by 
researchers more than any other few square km in the world.

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