[gdal-dev] Working with NED elevation data - part 2
Brent Fraser
bfraser at geoanalytic.com
Mon Aug 23 10:17:56 EDT 2010
Steve,
Excellent info! I had been contemplating using GDAL's VRT format to create
an elevation web service so I could display X,Y and Z on mouse moves in web
mapping, but was unsure about the performance. Looks like fcgi could work.
FYI, I think I'll use Google's <ElevationResponse> XML format
(http://code.google.com/apis/maps/documentation/elevation/), but I may extend to
have some kind of quality/accuracy and vertical datum values too. Here's a
stock Google example:
<ElevationResponse>
<status>OK</status>
<result>
<location>
<lat>39.7391536</lat>
<lng>-104.9847034</lng>
</location>
<elevation>1608.8402100</elevation>
</result>
<result>
<location>
<lat>36.4555560</lat>
<lng>-116.8666670</lng>
</location>
<elevation>-50.7890358</elevation>
</result>
</ElevationResponse>
Or perhaps the less verbose USGS format
(http://gisdata.usgs.gov/XMLWebServices2/Elevation_Service_Methods.php):
<Data_ID>[Source Layer ID]</Data_ID>
<Elevation>[Decimal Elevation Value]</Elevation>
<Units>[FEET or METERS]</Units>
Thanks again...
Best Regards,
Brent Fraser
Stephen Woodbridge wrote:
> 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 example
> C 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.html
> which is not otherwise referenced.
>
> Anyway, code is working great now. I am generating an elevation profile
> from a number of points along a path.
>
> 1. tried to get the points via USGS NED webservice: 132 point took 4-6
> min 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 compute
> a 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
>>>>
>>>>
>>>>
>>>> woodbri at 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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