Re: [gdal-dev] Working with NED elevation data - part 2

Stephen Woodbridge Sun, 22 Aug 2010 20:30:38 -0700

Even,

It seems the adding:


#include <cpl_conv.h>

resolves the segv issue.

I would recommend changing the example in:

http://www.gdal.org/gdal_tutorial.html

To include that in the section "Reading Raster Data" because the exampleC code uses CPLMalloc(). It would also be very helpful to add:


...
CPLFree(pafScanline);

to the example code. And a link to http://gdal.org/cpl__conv_8h.htmlwhich is not otherwise referenced.

Anyway, code is working great now. I am generating an elevation profilefrom a number of points along a path.

1. tried to get the points via USGS NED webservice: 132 point took 4-6min and 132 points is a short path.


So I downloaded the USGS NED data and created a .vrt file for the layer:

2. C code to open vrt file and handle requests via local CGI took 4 sec
3. C code to open vrt file and handle requests via local FCGI took < 1 sec

And each profile time above has some overhead because it has to computea route in pgrouting to get the points needed to generate the profile.


http://imaptools.com:8080/routeloops/route_loops6.php?x=-71.387764&y=42.620002&length=4828.032&heading=-1&shape=-1&rand=0.06593320066404917&allow=0%2C1&zoom=11&lat=42.62&lon=-71.38776&layers=B00TT&ll=-71.387764%2042.620002&len=3&rs=0.4646901273751023&al=0,1&u=E&cw=1&f=profile

Thanks for the help and support everyone.

-Steve

On 8/22/2010 12:43 PM, Even Rouault wrote:

Stephen,

at first sight, I don't see anything wrong with your code.

You need to include "cpl_conv.h" for CPLMalloc(). And memory allocated with
CPLMalloc() should be freed with CPLFree().  The doc is there :
http://gdal.org/cpl__conv_8h.html  (http://gdal.org/ ->  API reference
documentation ->  Files ->  cpl_conv.h )

But that doesn't likely account for the segmentation fault you have. This code
should work just fine with a GTiff or VRT dataset.

Do you use a recent version of GDAL ? If not, retry ;-) If still crashing,
could you attach a debugger and show the stack trace at the place where it
crashes and/or run valgrind on it ?

Best regards,

Even

Le dimanche 22 août 2010 05:23:45, Stephen Woodbridge a écrit :

OK, well may to answer my own question by reading the GDAL API Tutorial
and taking a guess at some parts I came up with the following. This is
basically just a copy of the tutorial and at the end I added some code
that I'm hoping reads the correct pixel for my lat,lon and prints out
the elevation.

The tutorial code for "Fetching a Raster Band" works on a GTiff file but
segv on a VRT file. Not sure why this happened, although there was a
compiler warning about:

testit.c: In function 'main':
testit.c:80: warning: cast to pointer from integer of different size

which would be:

      float *pafScanline;
      pafScanline = (float *) CPLMalloc(sizeof(float)*nXSize);

I could not find docs on CPLMalloc, or any info on if I need to free
that and how!

I would appreciate if someone could code review the code at the end and
comment on whether or not this is correct.

Thanks,
    -Steve

#include<gdal.h>

int main() {
      GDALDatasetH  hDataset;
      char *pszFilename = "/u/srcdata/ned2.vrt";
      //char *pszFilename = "/u/srcdata/NED2/-90_28_-84_30.tif";

      GDALDriverH   hDriver;
      double        adfGeoTransform[6];

      GDALRasterBandH hBand;
      int             nBlockXSize, nBlockYSize;
      int             bGotMin, bGotMax;
      double          adfMinMax[2];

      float *pafScanline;
      int   nXSize;

      int nXOff, nYOff;
      float elevation;

      float lon = -74.0;
      float lat =  42.0;

      GDALAllRegister();

      hDataset = GDALOpen( pszFilename, GA_ReadOnly );
      if( hDataset == NULL ) {
          printf("Failed to open %s!\n", pszFilename);
          exit(1);
      }

      hDriver = GDALGetDatasetDriver( hDataset );
      printf( "Driver: %s/%s\n",
          GDALGetDriverShortName( hDriver ),
          GDALGetDriverLongName( hDriver ) );

      printf( "Size is %dx%dx%d\n",
          GDALGetRasterXSize( hDataset ),
          GDALGetRasterYSize( hDataset ),
          GDALGetRasterCount( hDataset ) );

      if( GDALGetProjectionRef( hDataset ) != NULL )
          printf( "Projection is `%s'\n", GDALGetProjectionRef( hDataset )
);

      if( GDALGetGeoTransform( hDataset, adfGeoTransform ) == CE_None )
      {
          printf( "Origin = (%.6f,%.6f)\n",
          adfGeoTransform[0], adfGeoTransform[3] );

          printf( "Pixel Size = (%.6f,%.6f)\n",
              adfGeoTransform[1], adfGeoTransform[5] );
      }

      hBand = GDALGetRasterBand( hDataset, 1 );
      GDALGetBlockSize( hBand,&nBlockXSize,&nBlockYSize );
      printf( "Block=%dx%d Type=%s, ColorInterp=%s\n",
          nBlockXSize, nBlockYSize,
          GDALGetDataTypeName(GDALGetRasterDataType(hBand)),
          GDALGetColorInterpretationName(
              GDALGetRasterColorInterpretation(hBand)) );

      adfMinMax[0] = GDALGetRasterMinimum( hBand,&bGotMin );
      adfMinMax[1] = GDALGetRasterMaximum( hBand,&bGotMax );
      if( ! (bGotMin&&  bGotMax) )
          GDALComputeRasterMinMax( hBand, TRUE, adfMinMax );

      printf( "Min=%.3fd, Max=%.3f\n", adfMinMax[0], adfMinMax[1] );

      if( GDALGetOverviewCount(hBand)>  0 )
          printf( "Band has %d overviews.\n", GDALGetOverviewCount(hBand));

      if( GDALGetRasterColorTable( hBand ) != NULL )
          printf( "Band has a color table with %d entries.\n",
                   GDALGetColorEntryCount(
                       GDALGetRasterColorTable( hBand ) ) );

      /*
       * This section segv trying to do a memset in GDALRasterIO
       * if I use the VRT driver, but works OK on the GTiff file.
       */
      /*
      nXSize = GDALGetRasterBandXSize( hBand );

      pafScanline = (float *) CPLMalloc(sizeof(float)*nXSize);
      GDALRasterIO( hBand, GF_Read, 0, 0, nXSize, 1,
                    pafScanline, nXSize, 1, GDT_Float32,
                    0, 0 );
      */

      /* convert the lon, lat in a pixel location on the file and
       * read that location into a float.
       */
      nXOff = (lon - adfGeoTransform[0]) / adfGeoTransform[1] + 0.5;
      nYOff = (lat - adfGeoTransform[3]) / adfGeoTransform[5] + 0.5;
      GDALRasterIO( hBand, GF_Read, nXOff, nYOff, 1, 1,
                    &elevation, 1, 1, GDT_Float32,
                    0, 0 );
      printf("Elevation at %.6f, %.6f = %.1f\n", lon, lat, elevation);

      GDALClose(hDataset);
      return(0);
}

On 8/21/2010 6:07 PM, Stephen Woodbridge wrote:

So I have been able to download the NED data and I have US coverage.
The data is in GeoTiff and all in a single band Int16 format.
I have create a ned.vrt file.

So I'm interested in writing a simple program in C that I can pass a
lat, lon and get the elevation. I assume I need to open the .vrt file
but looking at the GDAL API, I'm not sure how to map the lat, lon to a
pixel and then how to fetch the pixel value. If it matter, I will
probably extend this later to process a list of locations or maybe
extend it to read a shapefile and process the shape through it.

Does anyone have a sample program that does something like this.

-Steve



wood...@mappy:/u/srcdata$ gdalinfo -stats ned2.vrt
Driver: VRT/Virtual Raster
Files: ned2.vrt
NED2/-100_25_-96_28.tif
NED2/-102_28_-96_30.tif
NED2/-102_30_-96_32.tif
NED2/-102_32_-96_34.tif
NED2/-102_34_-96_36.tif
NED2/-102_36_-96_38.tif
NED2/-102_38_-96_40.tif
NED2/-102_40_-96_42.tif
NED2/-102_42_-96_44.tif
NED2/-102_44_-96_46.tif
NED2/-102_46_-96_48.tif
NED2/-102_48_-96_50.tif
NED2/-108_28_-102_30.tif
NED2/-108_30_-102_32.tif
NED2/-108_32_-102_34.tif
NED2/-108_34_-102_36.tif
NED2/-108_36_-102_38.tif
NED2/-108_38_-102_40.tif
NED2/-108_40_-102_42.tif
NED2/-108_42_-102_44.tif
NED2/-108_44_-102_46.tif
NED2/-108_46_-102_48.tif
NED2/-108_48_-102_50.tif
NED2/-114_30_-108_32.tif
NED2/-114_32_-108_34.tif
NED2/-114_34_-108_36.tif
NED2/-114_36_-108_38.tif
NED2/-114_38_-108_40.tif
NED2/-114_40_-108_42.tif
NED2/-114_42_-108_44.tif
NED2/-114_44_-108_46.tif
NED2/-114_46_-108_48.tif
NED2/-114_48_-108_50.tif
NED2/-120_32_-114_34.tif
NED2/-120_34_-114_36.tif
NED2/-120_36_-114_38.tif
NED2/-120_38_-114_40.tif
NED2/-120_40_-114_42.tif
NED2/-120_42_-114_44.tif
NED2/-120_44_-114_46.tif
NED2/-120_46_-114_48.tif
NED2/-120_48_-114_50.tif
NED2/-123_33_-120_37.tif
NED2/-125_37_-120_39.tif
NED2/-125_39_-120_42.tif
NED2/-126_42_-120_44.tif
NED2/-126_44_-120_46.tif
NED2/-126_46_-120_48.tif
NED2/-126_48_-120_50.tif
NED2/133_6.5_138.5_10.tif
NED2/-139.0019842_51.9997277_-127.0018384_58.9998127.tif
NED2/-139.0024054_58.9997472_-127.0018101_66.0000944.tif
NED2/144_13_146_16.tif
NED2/151_5_163.5_8.tif
NED2/-157_18_-152_23.tif
NED2/-157.5_18.75_-154.5_21.5.tif
NED2/-160.5_21_-157.5_22.5.tif
NED2/-162_18_-157_23.tif
NED2/-171_-14.5_-169_-14.tif
NED2/-68_17_-64_18.tif
NED2/-72.0002778_40.9999994_-67.9999992_44.tif
NED2/-72.0002778_44_-66_46.tif
NED2/-72.0002778_46_-66_48.tif
NED2/-78.0002778_33.0000047_-75.0000047_36.tif
NED2/-78.0002778_36_-72_38.tif
NED2/-78.0002778_38_-72_40.tif
NED2/-78.0002778_40_-72_42.tif
NED2/-78.0002778_42_-72_44.tif
NED2/-78.0002778_44_-72_46.tif
NED2/-84.0002778_24_-78_26.tif
NED2/-84.0002778_26_-78_28.tif
NED2/-84.0002778_28_-78_30.tif
NED2/-84.0002778_30_-78_32.tif
NED2/-84.0002778_32_-78_34.tif
NED2/-84.0002778_34_-78_36.tif
NED2/-84.0002778_36_-78_38.tif
NED2/-84.0002778_38_-78_40.tif
NED2/-84.0002778_40_-78_42.tif
NED2/-84.0002778_42_-78_44.tif
NED2/-85.0002778_44.0000047_-82.0000047_47.tif
NED2/-90_28_-84_30.tif
NED2/-90_30_-84_32.tif
NED2/-90_32_-84_34.tif
NED2/-90_34_-84_36.tif
NED2/-90_36_-84_38.tif
NED2/-90_38_-84_40.tif
NED2/-90_40_-84_42.tif
NED2/-90_42_-84_44.tif
NED2/-90_44_-85_46.tif
NED2/-90_46_-85_49.tif
NED2/-96_28_-90_30.tif
NED2/-96_30_-90_32.tif
NED2/-96_32_-90_34.tif
NED2/-96_34_-90_36.tif
NED2/-96_36_-90_38.tif
NED2/-96_38_-90_40.tif
NED2/-96_40_-90_42.tif
NED2/-96_42_-90_44.tif
NED2/-96_44_-90_46.tif
NED2/-96_46_-90_48.tif
NED2/-96_48_-90_50.tif
Size is 1180816, 284173
Coordinate System is:
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,
AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0],
UNIT["degree",0.0174532925199433],
AUTHORITY["EPSG","4326"]]
Origin = (-171.000000000113090,66.000094447240443)
Pixel Size = (0.000283278659440,-0.000283278659440)
Corner Coordinates:
Upper Left (-171.0000000, 66.0000944) (171d 0'0.00"W, 66d 0'0.34"N)
Lower Left (-171.0000000, -14.5000520) (171d 0'0.00"W, 14d30'0.19"S)
Upper Right ( 163.4999735, 66.0000944) (163d29'59.90"E, 66d 0'0.34"N)
Lower Right ( 163.4999735, -14.5000520) (163d29'59.90"E, 14d30'0.19"S)
Center ( -3.7500132, 25.7500212) ( 3d45'0.05"W, 25d45'0.08"N)
Band 1 Block=128x128 Type=Int16, ColorInterp=Gray
Minimum=-83.000, Maximum=4665.000, Mean=25.333, StdDev=189.129
Metadata:
STATISTICS_MINIMUM=-83
STATISTICS_MAXIMUM=4665
STATISTICS_MEAN=25.332848474484
STATISTICS_STDDEV=189.12886049417
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Re: [gdal-dev] Working with NED elevation data - part 2

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