[Generate contours]Using fixed levels goes wrong

[meems@…]

We use the GDAL library in MapWindow (MW) and we found some inconsistancies with the gdal_contour function.
This function is wrapped in MapWindow in its Utils library and is used as:
u.GenerateContour(sGridFileName, sPolylinesFileName2, dInterval,
oGrid.Header.NodataValue, False, FixedLevels)
When I use this function in my VB6 application some grids create contourlines which are broken into several parts instead of being one long line.

First we tought it was a VB6 problem because when using the same grid and a VB.NET application the problem didn't occured.

But one of the other developers of MW has found out the same happens when using the gdal_contour command line tool.

So it looks like a genuine bug. Could someone look into it?

I've made also a bug report in the bugzilla of MW at:
http://bugs.mapwindow.org/show_bug.cgi?id=251
That report has not only more info but has also some sample data.

I hope someone can fix this.

With regards,
Paul

[meems@…]

Is this bug already scheduled to be fixed?
I'm waiting on it and I really whould like to know what the status is.

With regards.

Paul

[warmerdam]

Paul,

I've gone through #251, and grabbed the sample data, but I don't see the
specifics of what commandline arguments you used for gdal_contour?

I'm afraid I don't "schedule" non-customer bug reports.  I process them on a
random basis as time, energy, importance of bug, and simplicity of reproducing
merit.  Extra encouragement (like above) are also helpful in identifying stuff
that is really needed. 

[meems@…]

Thanks Frank for your swift response. 
I encountered the bug using your library in VB6 with MapWindow. Rob Cairns has found the same problem using the command line version of gdal_contour.
I've asked him what his setting were. As soon as I have them I'll let you know.

When I use my sample data in VB6 I get the error but when I use the same data in the MapWindow GUI (written in .NET) the problem doesn't occure. So I first thought it was my problem or MapWindows but because Rob got the same error I've posted it here.
If it is a bug of gdal and it can be fixed, my programm is working as it should be. If it's not a bug of gdal or it cannot be fixed I need to rewrite my programm to VB.NET.
As you can see that's is a big decision.

--
Paul

[iotc@…]

Hi Frank and Paul

Paul asked me to post the specific command line arguments. Using the sample data rad1E58F.asc downloaded from http://bugs.mapwindow.org/show_bug.cgi?id=251 they are:

gdal_contour -a elev -i 0.05 rad1E58F.asc contour_interval.shp
gdal_contour -a elev -fl 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 rad1E58F.asc contour_fixedlevel.shp 

contour_interval.shp is fine. contour_fixedlevel.shp has the broken contour line at the bottom of the picture as reported.

Regards Rob 

[meems@…]

Frank,

Can you reproduce this bug?

If needed I can give you more sample data.

Thanks,
Paul

[meems@…]

Frank,

I've been working on it and it seems I found the problem. When I use these fixed levels:
0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 
I get broken lines. When I zoom to the breakpoint and look at the gridvalue at that point the gridvalue equals a fixed level value. In this test I got two broken lines, both where the gridvalue was 1.5.
When I change the fixed levels to 
0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.4999 1.55 1.6 
I don't get broken lines.

For now this looks to be a usable work-around for me, but I need to do some more testing.

I hope this helps you also fixing the problem.

Paul

[Jukka Rahkonen]

This error happens still with GDAL 2.0-dev on Windows 7, 64-bit.