[GRASS-user] OFF-TOPIC - Earth Curvature in profiles

[Carlos "Guâno" Grohmann]

Well thanks all for the answers!

This is what I did:

- used r.profile to get (dist, elev) data from E-W profiles.
- I didn't considered the elipsoid, I used and approximation to a
sphere (to simplify my life :))
- from the total distance, I calculated the arc of curvature (theta)
- now I have a spreadsheet with my data (gnumeric)
- for the first point, I assigned a angular value of half the arc (theta/2)
- for each subsequent point, I subtracted the value of (theta/n) - for
n points in the profile
- then I created another column, with the "elev" value times 100 (for
vertical exaggeration) and added the radius of the Earth (6371 km)
- finally I created a polar plot of the distance and angles.

I not sure how "right" this is, but it seem to be a fair approximation.

Carlos



On Jan 15, 2008 3:30 AM, Hamish <hamish_b at yahoo.com> wrote:
> Carlos wrote:
> > Sorry for the OT, but can anyone point me some reference (paper, web,
> > etc) that explains how can I draw a topographic profile using the
> > Earth´s curvature? I am thiking about the output of r.profile (ascii,
> > distance from origin + elevation), in a section of about 3000 km,
> > with a vertical exaggeration of about 200 times.
>
>
> maybe a little work, but consider elevation as radius perturbation and
> distance along transect as theta-angle, then draw a circle with the
> relief shudder in it. ie like an AM radio wave.
>
> when that is working you can change the ideal circle to an ellipse of
> appropriate bend for 3000km.
>
>
> but probably it is better to just figure a way to calculate the
> vertical distance between a chord connecting the two 3000km ends and
> the circumference for each step along the transect.
>
> then add that distance to the elevation.
>
>
> just an idea,
> Hamish
>
>
>
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-- 
+-----------------------------------------------------------+
              Carlos Henrique Grohmann - Guano
  Geologist M.Sc  - Doctorate Student at IGc-USP - Brazil
Linux User #89721  - carlos dot grohmann at gmail dot com
+-----------------------------------------------------------+
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