[GRASS-user] GRASS & earth curvature correction (viewsheds, LOS)

GRASS GRASS user
1

[snip]

>
> However, after some testing and comparison with
> the output of ArcGIS' Viewshed module, I can now
> confirm that the correction method works just fine.

It gives confidence sure, but there's no way of knowing if their
black box is actually correct or not, maybe we make the same
mistakes? :slight_smile: All the better to follow/cite published articles.

Of course, that will always be a problem. But I have
compared three independent methods: r.los with built-in
correction, ArcGIS with built-in correction, and r.ecurv.comp.
In general, the outputs are very similar, even down to
small detail. The remaining differences must be in
implementation details and the fact that r.los does
not implement atmospheric refraction.

I have also run some of the LOS algorithms in SEXANTE
(on a DEM that was processed with r.ecurv.comp, since
they lack any built-in correction). Again, results
were very much the same as r.los and ArcGIS.

Only ArcGIS outputs those contiguous viewsheds, though.
So they must have done something to tune the output.

certainly the ideal solution is to have both curvature and
refection corrections implemented as flags in the new & improved
r.viewshed. (say, is that now ready for moving into grass7? *)

That should be really easy to do. All that's needed is to
amend the existing correction but taking away 1/7 to account
for the adverse effect of refraction.

Does refraction only work in the vertical or would you be able
to slightly see around some distant volcano horizontally?
..perhaps the phenomenon is related to the atm pressure
gradient, in which case purely a vertical-only effect..?

Well, that refraction correction is really a rough
simplification of reality. Essentially, it uses the same
amount of correction as ArcGIS. There is some justification
for this. You can find links to articles here:

http://mapaspects.org/content/effects-curvature-earth-refraction-light-air-and-fuzzy-viewsheds-arcgis-92

(@Markus N: maybe that answers your question, as well?)

But in summary, accounting for realistic refraction conditions
would be much more complex, as it would also have to take into
plus different refraction at different elevations, etc.

But given that most DEMs have an inherent vertical error that
is greater than the influence of these effects, I am not sure
it's worth spending too much time on (it might be for very
long distance visibility -- I just don't know).

Do you think that noise happens because of a translation of the
coarse horizontal grid cell size when it is translated into the
vertical plane?

That's a really interesting thought.
It could very well be a quantization effect of that kind.
That might explain why the noisy patches seem to show
structural features of the DEM.

Cheers,

Ben

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2

Hamish wrote:
[...]

> the ideal solution is to have both curvature and refection
> corrections implemented as flags in the new & improved
> r.viewshed. [...]

Benjamin wrote:

That should be really easy to do. All that's needed is to
amend the existing correction but taking away 1/7 to
account for the adverse effect of refraction.

ok, done for r.viewshed in r46423. Number of visible cells
reduces slightly when the curvature flag is used, and rebounds
ever so slightly when the refraction flag is used.
Please test.

Well, that refraction correction is really a rough
simplification of reality. Essentially, it uses the same
amount of correction as ArcGIS. There is some justification
for this. You can find links to articles here:
http://mapaspects.org/content/effects-curvature-earth-refraction-light-air-and-fuzzy-viewsheds-arcgis-92

I could not get at the Yoeli(1985) article as it's behind a
paywall my univ does not subscribe to. Can anyone say what's in
it?

But in summary, accounting for realistic refraction conditions
would be much more complex, as it would also have to take
into plus different refraction at different elevations, etc.

I don't mind that / it is not so different from the physics I do
in my day job, and just using a fudge factor of +1/7th leaves me
feeling like the job is poorly done. Passing the coeff off to the
user without further guidance seems like a bit of a cop out. I
suppose there is a gradient in the coeff as you move from the
tropics to high latitudes, daily temperature, Linke factor,
humidity, aerosols, etc ... ?

But given that most DEMs have an inherent vertical error that
is greater than the influence of these effects,

can we quantify that? for example what's STRM 95% confidence
accuracy?

I am not sure it's worth spending too much time on (it might
be for very long distance visibility -- I just don't know).

it would be good for us to do a rough back of the envelope calc
to justify that before fully forgetting about it.

I guess for the worst case scenario we could try the views from
Mt. Everest and/or Olympus Mons and see what difference it makes.

thanks,
Hamish

3

On Thu, May 26, 2011 at 10:50 AM, Hamish <hamish_b@yahoo.com> wrote:

Hamish wrote:
[...]

> the ideal solution is to have both curvature and refection
> corrections implemented as flags in the new & improved
> r.viewshed. [...]

Benjamin wrote:

That should be really easy to do. All that's needed is to
amend the existing correction but taking away 1/7 to
account for the adverse effect of refraction.

ok, done for r.viewshed in r46423. Number of visible cells
reduces slightly when the curvature flag is used, and rebounds
ever so slightly when the refraction flag is used.
Please test.

Well, that refraction correction is really a rough
simplification of reality. Essentially, it uses the same
amount of correction as ArcGIS. There is some justification
for this. You can find links to articles here:
http://mapaspects.org/content/effects-curvature-earth-refraction-light-air-and-fuzzy-viewsheds-arcgis-92

Hm-hm. Citing from the website:
"The problem is that the ratio of change due to air to curvature is
not 1:7 (0.13), as the standard refraction coefficient suggests. It is
0.325.

Last spring I put together an Excel sheet that computes this ratio.
Having the adjustable details (altitude, air pressure, wavelength,
etc.) did show me that the ratio never really changes (given earthly
conditions). What it did show me was that the ratio was always 0.325."

[...]

But given that most DEMs have an inherent vertical error that
is greater than the influence of these effects,

can we quantify that? for example what's STRM 95% confidence
accuracy?

From Farr et al. 2007:

Summary of SRTM performance. All quantities represent 90% errors in meters.

  Africa Australia Eurasia Islands N. America S. America
Absolute Geolocation Error 11.9 7.2 8.8 9 12.6 9
Absolute Height Error 5.6 6 6.2 8 9 6.2
Relative Height Error 9.8 4.7 8.7 6.2 7 5.5
Long Wavelength Height Error 3.1 6 2.6 3.7 4 4.9

[sorry for the ugly format, it's tab separated]

I am not sure it's worth spending too much time on (it might
be for very long distance visibility -- I just don't know).

it would be good for us to do a rough back of the envelope calc
to justify that before fully forgetting about it.

I guess for the worst case scenario we could try the views from
Mt. Everest and/or Olympus Mons and see what difference it makes.

No need to go into mountains, just increase observer elevation offset,
preferably in a moderately flat area to get really far views. Using
correction for earth curvature only, max is a bit more than 100 km
with 3km observation offset. 200km is impossible without leaving
earth's atmosphere.

Markus M

4

----- Original Message -----

Hamish wrote:

[..]

ok, done for r.viewshed in r46423. Number of visible cells
reduces slightly when the curvature flag is used, and rebounds
ever so slightly when the refraction flag is used.
Please test.

Cool, that's what one would expect. Reassuring.

> Well, that refraction correction is really a rough
> simplification of reality. Essentially, it uses the same
> amount of correction as ArcGIS. There is some justification
> for this. You can find links to articles here:
> http://mapaspects.org/content/effects-curvature-earth-refraction-light-air-and-fuzzy-viewsheds-arcgis-92

I could not get at the Yoeli(1985) article as it's behind a
paywall my univ does not subscribe to. Can anyone say what's in
it?

> But in summary, accounting for realistic refraction conditions
> would be much more complex, as it would also have to take
> into plus different refraction at different elevations, etc.

I don't mind that / it is not so different from the physics I do
in my day job, and just using a fudge factor of +1/7th leaves me
feeling like the job is poorly done. Passing the coeff off to the
user without further guidance seems like a bit of a cop out. I
suppose there is a gradient in the coeff as you move from the
tropics to high latitudes, daily temperature, Linke factor,
humidity, aerosols, etc ... ?

I am sure there is. But I lack the background to judge this
correctly.

> But given that most DEMs have an inherent vertical error that
> is greater than the influence of these effects,

can we quantify that? for example what's STRM 95% confidence
accuracy?

[I think this needs a probabilistic approach, see my other
reply to this thread.]

> I am not sure it's worth spending too much time on (it might
> be for very long distance visibility -- I just don't know).

it would be good for us to do a rough back of the envelope calc
to justify that before fully forgetting about it.

I guess for the worst case scenario we could try the views from
Mt. Everest and/or Olympus Mons and see what difference it makes.

-- that would rock :slight_smile:

Ben

thanks,
Hamish

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