[gdal-dev] Puzzling results from gdalinfo with BSB nautical charts

[Jack Kirkpatrick]

Greetings,

I have been exploring the GDAL utilities with a view to creating a general BSB to RLE bitmap converter that also generates a separate WGS84 worldfile, appropriately transformed for the datum etc of the original BSB file.

I believe I am getting some puzzling results from the gdalinfo utility that I will try to explain as follows:

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(1) Environment is Windows XP SP3 and I have installed FWTools 2.2.9.


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(2) I have obtained NOAA BSB chart no 11006 (KEY WEST TO THE MISSISSIPPI RIVER) from:

http://www.nauticalcharts.noaa.gov/mcd/Raster/download_agreement.htm

scroll to the bottom then click [Proceed to NOAA RNC(R) (BSB) Downloads] then click [Textual], the rest is pretty obvious.


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(3) After downloading and unzipping to [C:\NOAA BSB\11006], I inspect the file [C:\NOAA BSB\11006\11006_1.KAP] with the Windows Wordpad applet, the first several lines read as follows:

!
CRR/
VER/3.0
BSB/NA=KEY WEST TO THE MISSISSIPPI RIVER,NU=44,RA=10967,9389,DU=254
KNP/SC=875000,GD=NAD83,PR=MERCATOR,PP=26.127,PI=60.000,SP=,SK=0.0000000
TA=90.0000000,UN=FATHOMS,SD=MEAN LOWER LOW WATER,DX=87.50,DY=87.50
...

I'm guessing "GD=NAD83,PR=MERCATOR" means the file contents are to NAD83 datum in a Mercator projection.

This file also contains 92 ground control points of the form:

REF/1,275,8522,24.1668694444,-90.0831111111
REF/2,275,300,30.5834527778,-90.0831111111
REF/3,2427,300,30.5834527778,-88.1999805556
...
REF/92,275,1069,30.0000583333,-90.0831111111

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(4) I launch the FWTools shell via the [FWTools Shell] shortcut on the desktop and within the FWTools shell I key:

gdalinfo -nomd -noct "C:\NOAA BSB\11006\11006_1.kap" > "C:\NOAA BSB\11006\11006_1.txt"


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(5) I inspect the resulting file [C:\NOAA BSB\11006\11006_1.txt] which shows:

Driver: BSB/Maptech BSB Nautical Charts
Files: C:\NOAA BSB\11006\11006_1.kap
Size is 10967, 9389
Coordinate System is `'
GCP Projection = GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG",7030]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG",6326]],PRIMEM["Greenwich",0,AUTHORITY["EPSG",8901]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG",9108]],AXIS["Lat",NORTH],AXIS["Long",EAST],AUTHORITY["EPSG",4326]]
GCP[ 0]: Id=GCP_1, Info=
(275,8522) -> (-90.0831111111,24.1668694444,0)
GCP[ 1]: Id=GCP_2, Info=
(275,300) -> (-90.0831111111,30.5834527778,0)
GCP[ 2]: Id=GCP_3, Info=
(2427,300) -> (-88.1999805556,30.5834527778,0)
...
GCP[ 91]: Id=GCP_92, Info=
(275,1069) -> (-90.0831111111,30.0000583333,0)
Corner Coordinates:
Upper Left ( 0.0, 0.0)
Lower Left ( 0.0, 9389.0)
Upper Right (10967.0, 0.0)
Lower Right (10967.0, 9389.0)
Center ( 5483.5, 4694.5)
Band 1 Block=10967x1 Type=Byte, ColorInterp=Palette
Color Table (RGB with 12 entries)

<<<<< i.e. somehow gdalinfo has decided that the files contents are to WGS84 datum >>>>>

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(6) I rummaged through the gdal source and found [C:\gdal-1.5.3\frmts\bsb\...] which appears to contain the source files for the GDAL BSB driver(?). 

Of particular interest is the file [C:\gdal-1.5.3\frmts\bsb\bsbdataset.cpp] which contains the line:

pszGCPProjection = "GEOGCS[\"WGS 84\",DATUM[\"WGS_1984\",SPHEROID[\"WGS 84\",6378137,298.257223563,AUTHORITY[\"EPSG\",7030]],TOWGS84[0,0,0,0,0,0,0],AUTHORITY[\"EPSG\",6326]],PRIMEM[\"Greenwich\",0,AUTHORITY[\"EPSG\",8901]],UNIT[\"degree\",0.0174532925199433,AUTHORITY[\"EPSG\",9108]],AXIS[\"Lat\",NORTH],AXIS[\"Long\",EAST],AUTHORITY[\"EPSG\",4326]]";

which is a dead ringer for the [GSP Projection =...] produced by gdalinfo.

I am not a C programmer but reading [C:\gdal-1.5.3\frmts\bsb\bsbdataset.cpp] and [C:\gdal-1.5.3\frmts\bsb\bsb_read.c] it looks to me suspiciously like the GDAL BSB driver forces the gdalinfo datum to WGS84 regardless of what it truly is.

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(7) I am led to believe that NAD83 is to all intents and purposes WGS84 so I guess there is no harm done if all BSB charts were NAD83 or similarly "WGS84 compatible". 

However if you go to:

http://www.digitalocean.ca/digitaloceane-store/bsbraster.asp

you can obtain BSB charts for areas totally divorced from NOAA (and therefor presumably NAD83 and similar).

My concern is that I could be in the situation of trying to use the GDAL utilities to convert (say) some New Zealand BSB charts that use a datum that is not "WGS84 compatible" but the GDAL utilities force the datum to WGS84 without any appropriate transformation.

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Any and all comments welcomed - I do hope I haven't missed something embarrassingly simple in this great monologue...

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