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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | RE: [gislist] US cities and counties |
| Date: |
03/24/2004 11:15:01 AM |
| From: |
Martin du Saire |
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Wow, fascinating stuff. I better stop now before I get sucked in
entirely. I cc to the list in case anyone was following this thread.
Thanks again.
Martin
At 12:02 PM 3/24/2004 -0500, you wrote:
>Martin,
>At 10:03 AM 3/24/2004 -0600, you wrote:
>> How do you go about selecting a projection, trial and error?
>
>Good question! Trial and error is too hard: there are hundreds of
>available projections and most can be modified in infinitely many
>ways. You need to have a basic understanding of the kinds of projections
>and their properties. Having a tool to evaluate any particular
>projection, like my "Tissot" extension for ArcView, can help to refine the
>initial choice. There's a nice introduction to this topic at
>http://hcgl1.eng.ohio-state.edu/~yi/map636/group61.htm . John Snyder's
>"An Album of Map Projections" (USGS Professional Paper 1453) describes and
>illustrates about a hundred projections: its purpose is to help people
>select map projections. It contains a two-page introduction, "Guide to
>Selecting Map Projections", that covers much of the same material as the
>Ohio State Web page.
>
>>the software I am using (GRASS5.3) supports Chamberlin Trimetric, for
>>example. Is this any better or worse than the abovementioned?
>
>ArcView does not support this so I cannot analyze it using my Tissot
>tool. Snyder's book describes it as a modified azimuthal projection,
>neither conformal nor equal area, with no symmetry and "complex curves"
>for meridians and parallels. The scale is "approximately correct from
>three selected widely spaced points on map to any other point on map." It
>has "low distortion throughout map if three source points are properly
>placed near map limits," but we are not told how low the distortion is,
>nor how to "properly" place the points. Another book of Snyder's ("Map
>Projections--A Working Manual") intimates this is a perturbation of an
>azimuthal equidistant projection, but does not provide details. For a
>picture of the graticule see
>http://www.ilstu.edu/microcam/map_projections/Azimuthal/Chamberlin.pdf
>. A brief description of its method of construction appears at
>http://www.ncgia.ucsb.edu/projects/tobler/Projections/sld028.htm . It
>appears simple enough to construct that you could probably analyze its
>behavior using a spreadsheet application.
>
>--Bill Huber
>
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