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Re: terrain normalisation
| Re: terrain normalisation [message #20492] |
Fri, 30 June 2000 00:00  |
Marcel Droz
Messages: 3
Registered: June 2000
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Junior Member |
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Thanks a lot for your explanations! I actually see the problem...but as
our studies will cover the whole alps - means from France to Austria...
- its rather difficult to find a <b>homogenous</b> DEM of the whole region,
especially with limited finacial resources.
<br>Do you know any organisation, research institute or whatever, who deals/dealt
with this kind of DEM (about 250m resolution or even better)?
<p>Thanks in advance, Marcel.
<p>richard hilton wrote:
<blockquote TYPE=CITE>I would be very careful with calculating the aspect
of a 1km DEM when you
<br>are trying to use a 1km AVHRR image for sun angle/aspect/slope calculations.
<br>You need to use a DEM that is of much higher resolution than that.
You only
<br>have "mean" height values for the 1km squares and not any information
about
<br>the slope/aspect. You can infer a slope/aspect from the surrounding
pixels
<br>but this can result in increasing the errors in using this value instead
of
<br>assuming that the alps are flat!!!!! (the information from the surrounding
<br>pixels cannot give subpixel infomation about what is happening inside
the
<br>central pixel.)
<p>eg.
<br> 10 10
10
<br> 10 5
10
<br> 10 10
10
<p>A 3x3 grid. you would assume that the mean slope/aspect for the central
<br>square are both 0 but.....
<p> 10 &nbs p;
10 &nbs p; 10
<br> 10 10
5   ; 0 10
<br> 10 &nbs p;
10 &nbs p; 10
<p>in this case there is a linear slope to the right (the extra 10 and
0 are
<br>the subpixel heights in the inner square in the extreme left and right
of
<br>the square)
<p>but in this case the slope is reversed:
<p> 10 &nbs p;
10 &nbs p; 10
<br> 10 0   ;
5   ; 10 10
<br> 10 &nbs p;
10 &nbs p; 10
<p>this is obviously a simple case but I hope you can see the difference.
If
<br>you are trying to calculate the sun angle then assuming the area to
be flat
<br>is potentially going to be more accurate (even in a mountainous area!!!).
<p>I think that you really need to be using a far heigher resolution DEM.
and
<br>then rebin the slope/aspects up to the 1km required resolution.
<p>We are currently doing a lot of research into existing 1km (30 arc-second
to
<br>be precise) DEMs (and creating our own, called ACE (Altimeter Corrected
<br>Heights)) in particular GLOBE_v1, GTOPO30 and the 5 arc-minute JGP95E
and
<br>have shown errors in heights of up to 1500m over parts of the world.
I must
<br>however stress that I haven't looked at the swiss alps. I've had a
look at
<br>the area, and the source data in GTOPO30 is DTED data. This is the
best type
<br>of data in the DEM, but this uses a 3 arc second pixel to represent
the
<br>whole 30 arc-second region and is not a mean over the whole area. This
gives
<br>vertical errors (according to GLOBE_v1) of 18-120m (they used the same
data
<br>but the stats are far more acuarate!!) What affect this will have on
a
<br>calculation of the slope/aspect is difficult to assess but I assume
it would
<br>be fairly devastating!!
<p>General features (rivers etc. always appear in the correct place
<br>(horizontally but not vertically) but the topography is often supersampled
<br>100-200m contours which gives rise to mathematical features and NOT
true
<br>representations of the land surface. There should be accurately surveyed
<br>maps of the swiss alps available since it is a developed country and
the
<br>surveying has probable been done very well, but how much these cost
and who
<br>owns them I'm afraid I don't know. I imagine that even a 50m resolution
DEM
<br>for a relatively small area would be incrediably expensive.
<p>I hope this helps
<p>Let me know if you want anymore information.
<p>Richard</blockquote>
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