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2004 May 04 18:10

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Re: [vox-tech] Has anyone done cartography on Linux?
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Re: [vox-tech] Has anyone done cartography on Linux?



On Tue, 4 May 2004, Peter Jay Salzman wrote:

> hi henry,
> 
> sounds like a 4-6 hour SDL programming project (2-3 hours for bill).
> 
> collect a bunch of mouse clicks, and it's not too difficult to calculate
> a perimeter using the coordinates, assuming you only have piecewise
> straight lines and a scaling.  just the good old pythagorean theorem
> (except for the degenerate vertical line case).

A = 0.5 * sum( Vn cross Vn+1 ) for n=0 to n-1
where point n is the same as point 0

Cross product is x_n * y_n+1 - x_n+1 * y_n

> i took graduate differential geometry with dr. thurston at UCD.  one of
> the things i learned (he was surprised that nobody in the class knew
> this, but i don't know where we'd see it) was that you can calculate the
> area of an arbitrary n sided polygon (an n-gon) using a rank n
> determinant.
> 
> i forget the exact details, but it shouldn't be too hard to find.
> 
> what else would you need?
> 
> there's nothing out there that does this?

That is what bugs me about this... these are trivial things to do, but I
don't see how to make Gimp do them.

ImageJ [1] calculates areas and lengths in pixels.  Visio does it.  I
don't really spend enough time in graphics programs doing this kind of
thing to be the person to say which programs are the best at it, though.

[1] http://rsb.info.nih.gov/ij/

> pete
> 
> 
> On Tue 04 May 04,  1:49 PM, Henry House <hajhouse@houseag.com> said:
> > Has anyone here worked with maps on Linux? I periodically need to work with
> > simple maps and I am continually frustrated with the tools that I have
> > tried. Here is an example of something that I want to do:
> > 
> > 1. Scan an aerial photo or existing map
> > 2. Mark some key points on it, connect them with lines or curves, make
> >    areas, etc. These might be fields or fences.
> > 3. Calculate lengths and areas (for example, number of acres in a field or
> >    number of miles long that a fence extends).
> > 
> > I don't have databases of geospacial data or GPS receiver data, so I don't
> > think a heavyweight GIS system what I want. The scales are normally small so
> > map projections are not a major issue. Anyway, what I want to do is so
> > simple that I don't think it is worth learning a complex system like GRASS
> > (<http://packages.debian.org/testing/science/grass>) since I can do a crude
> > job with pencil, ruler, and compensating polar planimiter* in not much time.
> > Still, there is surely a better way. I am interested in any suggestions.
> > 
> > * A mechanical device that, when adjusted for scale, calculates the area of
> >   any shape that one traces on a map or photograph.
> > 
> > -- 
> > Henry House
> > The unintelligible text that may follow is a digital signature.
> > See <http://hajhouse.org/pgp> to find out how to verify it.
> > My OpenPGP key: <http://hajhouse.org/hajhouse.asc>.
> > 
> 
> 
> 
> -- 
> Make everything as simple as possible, but no simpler.  -- Albert Einstein
> GPG Instructions: http://www.dirac.org/linux/gpg
> GPG Fingerprint: B9F1 6CF3 47C4 7CD8 D33E 70A9 A3B9 1945 67EA 951D
> _______________________________________________
> vox-tech mailing list
> vox-tech@lists.lugod.org
> http://lists.lugod.org/mailman/listinfo/vox-tech
> 

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