Generalize in GPSMapEdit

[Tsnake]

I tried using the Generalize feature under the Tools menu on GPS MapEdit.  I noticed on one of my quads it removed like 1E6 points and halved the file size.  Extactly what is it doing and does it affect the accuracy of my maps.

Thanks,
Sam

[-Oz-]

removed 1 million points; thats a lot.

I have never used it.  Anyone else?

[Tsnake]

Just tried it again.  10 million not 1 million.  My bad.  This is for a quad in southern Illinois.  The contours are set at 10, 50 and 100 ft.  Plus I have water data and road data.  The contours are extended up to levels 0, 1 and 2 respectively.  Maybe this is my mistake.  When I tried to run it in cgpsmapper, I got an error that said the RGN file was too large.

Sam

[-Oz-]

Ah yes, that error.  When I get that I do these two things:
- lower the RGN limit number in the .mp file to 800; then 400; if that doesn't work then I:
- split the quad in half and see if it'll process that way.

That was the advice from Stan (cgpsmapper's creator).

[erik.the.awful]

Yes I have used generalize as well as merge features and it cut down my 1.8GB NHD data to only 300 MB. Does this effect accuracy at all?

[-Oz-]

I would guarantee it does.

The question is does it affect it enough to notice.  I'm sure that it takes things and smooths them out or remove points to try and save space.  Similar to how MapSource can compress the number of points in a track; you lose accuracy but will you notice.

I can't answer the question though, it'd have to be trial and error on a smaller scale.

[Tsnake]

I tried it with a smaller file.  What I found is that, for example, you have a line made up of three points, the two ends and one in the middle that is slightly out of line.  The generalize function will remove that middle point and make a straight line.  This is especially noticable on contour lines.  On the small files I tried, it looked like the error was ~ 5 feet.  This is not a big deal on contour lines, but I did notice it happened to some roads. 

I have a feeling that cgpsmapper does the same thing when the generalize and intersection is selected for preprocessing.  However, if I went with no preprocessing to save the accuracy, not all of the interections on roads would meet.

Sam