[gdal-dev] Is there a reference documentation for GDALMetadata structure?

Mon May 27 10:17:24 PDT 2019

Hi Andrea,

I have understood that PAM may contain any metadata that cannot be stored into the file format itself.

I made a test but I honestly do not know what it proves, if anything.  However, if I tell GDAL to handle GeoTIFF as read-only and make some edits with

gdal_edit -ro -mo DATUM=WGS84 test.tif
gdal_edit -ro -scale 299 -offset 10 test.tif

then I get a tif.aux.xml file that contains

<PAMDataset>
  <Metadata>
    <MDI key="DATUM">WGS84</MDI>
  </Metadata>
  <PAMRasterBand band="1">
    <Offset>10</Offset>
    <Scale>299</Scale>
  </PAMRasterBand>
  <PAMRasterBand band="2">
    <Offset>10</Offset>
    <Scale>299</Scale>
  </PAMRasterBand>
  <PAMRasterBand band="3">
    <Offset>10</Offset>
    <Scale>299</Scale>
  </PAMRasterBand>
</PAMDataset>

If I run the commands without -ro then the tags of the GeoTIFF file are edited and no sidecar file appears.

-Jukka-

Lähettäjä: Andrea Aime <andrea.aime at geo-solutions.it>
Lähetetty: maanantai 27. toukokuuta 2019 18.59
Vastaanottaja: Rahkonen Jukka (MML) <jukka.rahkonen at maanmittauslaitos.fi>
Kopio: gdal-dev at lists.osgeo.org
Aihe: Re: [gdal-dev] Is there a reference documentation for GDALMetadata structure?

Hi Jukka,
thank you for following up. I believe that provides information for the PAM dataset, which
is normally a sidecar containing statistical information such as min/max/histograms.
The GDAL metadata TIFF tag instead normally contains other information, such as scale/offset,
band names, units, and uses a XML that has a different structure.

Looking into the code there is something around here:
https://github.com/OSGeo/gdal/blob/master/gdal/frmts/gtiff/geotiff.cpp#L13726

I see explicit treatment for scale/offset and a few others, not sure if there is anything else?
Like, if a band had a custom name, would it show up in the GDALMetadata structure in a significant way?
From my previous example, things like:

 <Item name="Band_1">Max Band_1</Item>.
    <Item name="Band_2">Min Band_1</Item>.
    <Item name="Band_3">Mean Band_1</Item>.
    <Item name="Band_4">SD Band_1</Item>.
    <Item name="Band_5">MASD Band_1</Item>.
    <Item name="Band_6">Valid Inputs</Item>.

are they actually used or just treated as "random strings"?
Looking at a GDALInfo output it looks like of the second, but maybe I'm missing something:

Metadata:
  AREA_OR_POINT=Area
  Band_1=Max Band_1
  Band_2=Min Band_1
  Band_3=Mean Band_1
  Band_4=SD Band_1
  Band_5=MASD Band_1
  Band_6=Valid Inputs
  UNITS=m
...
Band 1 Block=10x10 Type=Int32, ColorInterp=Gray
  NoData Value=-9999
  Offset: 0,   Scale:0.0001
Band 2 Block=10x10 Type=Int32, ColorInterp=Undefined
  NoData Value=-9999
  Offset: 0,   Scale:0.0001
Band 3 Block=10x10 Type=Int32, ColorInterp=Undefined
  NoData Value=-9999
  Offset: 0,   Scale:0.0001
Band 4 Block=10x10 Type=Int32, ColorInterp=Undefined
  NoData Value=-9999
  Offset: 0,   Scale:0.0001
Band 5 Block=10x10 Type=Int32, ColorInterp=Undefined
  NoData Value=-9999
  Offset: 0,   Scale:0.0001
Band 6 Block=10x10 Type=Int32, ColorInterp=Undefined
  NoData Value=-9999
  Offset: 0,   Scale:0.0001

Cheers
Andrea

On Mon, May 27, 2019 at 1:11 PM jratike80 <jukka.rahkonen at maanmittauslaitos.fi<mailto:jukka.rahkonen at maanmittauslaitos.fi>> wrote:
Hi,

Perhaps here
https://github.com/OSGeo/gdal/blob/master/gdal/gcore/gdalpamdataset.cpp

-Jukka Rahkonen-

geowolf wrote
> Hi,
> I was wondering if there is any documentation on what a GDALMetadata XML
> structure might contain, both in terms
> of element/attributes, and key names?
> ...

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