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Author Topic: Why not use GM for contours?  (Read 3179 times)

maps4gps

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Why not use GM for contours?
« on: December 02, 2008, 06:47:21 AM »
Oz,
  What is your reason for not using GM for contouring?


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Why not use GM for contours?
« Reply #1 on: December 02, 2008, 05:54:51 PM »
I wrote a quick batch file that will process an entire directory and use gdal_contour (part of fwtools) so create the contours.  It just allows me to start it running and then walk away.
Dan Blomberg
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maps4gps

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Why not use GM for contours?
« Reply #2 on: December 03, 2008, 07:05:22 AM »
I do the same with a script file in GM.  I was curious to know if you found a program that contoured without creating those unslightly short 2 point line segments.  Sorry to be off-topic; I have not mastered how to send an email to you on your site.

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Re: Why not use GM for contours?
« Reply #3 on: December 03, 2008, 08:43:31 AM »
I split it into a new topic since its a good discussion.

What short 2 point line segments? (can you take and post a screenshot)  Not sure I've seen them (or if I have it hasn't registered).  My new method was used to create Idaho and New Mexico topos.

I also did a speed test and the gdal_contour was faster only because I didn't have to wait for the GUI to load the data then process it.  It seemed like it removed a step.

If you're use a script file does it load the file then process it or does it just start processing it?
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maps4gps

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Re: Why not use GM for contours?
« Reply #4 on: December 03, 2008, 11:41:58 AM »
I do not have anything on the computer for a quick screenshot in GM.  However, using GPSMapEdit found at:
NM 54881491 file, 6400ft @ 34 19.521 & 106 00.267
WY 88201003 file, 9840ft @ 44 57.464 & 109 33.389
CO 88200004 file, 7920ft @ 40 42.374 & 107 07.815
also in the ME data

They are much easier to find in GM with just contour lines and not having to zoom in to have the small features show.  I suspect it is caused by two adjacent grid points having an elevation which is just a tiny amount on the other side of a contour interval than the other points around the area.
Probably a somewhat similiar reason for the small triangles.

Yes, a GM script file can process and output data without displaying it on the screen (much faster that way for huge files).  It does take time to load the file(s), it just does not require the additional time to display the data before processing (ie. contouring) or outputing (ie mp files).

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Re: Why not use GM for contours?
« Reply #5 on: December 04, 2008, 08:07:21 AM »
Yea, I've checked those spots.

I don't think it matters what software compiles the contour lines they will all likely have that issue.  Either because of the reason you mentioned above or just because the tif image shows a change (error or not).

It doesn't surprise me though if it were right becuse there are definitely some "holes" in Arizona that were pretty deep and large.

If its happening in Maine too its definitely any software because Maine was done differently than the others.
Dan Blomberg
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anything