[mapserver-users] WCS Tile index question

[Rahkonen Jukka (MML)]

Lähettäjä: MapServer-users <mapserver-users-bounces at lists.osgeo.org> Puolesta Carl Godkin



>> On Tue, Oct 19, 2021 at 5:37 PM Rahkonen Jukka (MML) <mailto:jukka.rahkonen at maanmittauslaitos.fi> wrote:
>> Hi,
 
>> Here is more accurate documentation but only in the text 
>> https://www.mapserver.org/ogc/wcs_server.html
 
>> “The convention is that once (wcs|ows)_extent and one of 
>> (wcs|ows)_size and (wcs|ows)_resolution is set in the layer metadata, all the coverage specific metadata will be retrieved from there. Otherwise the source image is queried via GDAL, if possible.”
 
>> It seems that only wcs_extent is documented in the list of metadata elements but this is what they do:
>> • wcs_extent defines the bounding box or your coverage -> that will 
>> go into DescribeCoverage • wcs_size is the size of the coverage as 
>> pixels -> pixel size can be computed by extent and size • 
>> wcs_resolution tells the pixel size explicitly -> size of the 
>> coverage in pixels can be computed

> Thanks a lot, Jukka.  That all makes some sense as far as it goes, but 
> I'm still not clear on a few details. I have this range of data: 24N to 50N and 125W to 66W which is 26 degrees by 59 degrees.

>  Since the SRTM data has 3 arc-second spacing, that's 1200 pixels per 
> degree plus one for the extra edge so I have tried a number of things without success.

What fails? Don't you get anything into DescribeCoverage or is all that you get wrong?

> 1. Is wcs_size the size FULL range of the tiles that I have?  And what is the order?

You do not need to bother about the size if you set the resolution, but yes, it is the full range of the coverage. See this https://beta-karttakuva.maanmittauslaitos.fi/wcs/service/ows?service=WCS&REQUEST=DescribeCoverage&VERSION=2.0.1&COVERAGEID=ortokuva__ortokuva

The size is converted into the pixel space of the coverage gml:GridEnvelope>
<gml:low>0 0</gml:low>
<gml:high>1348356 2316499</gml:high>
</gml:GridEnvelope

I would say that the order is left-to-right - down-to-up but I fear that by the GML standard it can be something else. You can read about rectified grid from the OpenGIS Geography Markup Language (GML) Encoding Standard 3.2.1. I have never understood it but there are some images which may clarify this: "rectified grid: grid for which there is an affine transformation between the grid coordinates and the coordinates of an external coordinate reference system"

> 2. Again, what is the order for wcs_resolution?  Are the units degrees since the coordinate system of the original data is EPSG:4326?
With Mapserver the order is easting/longitude - northing/latitude

> I've tried all of these, one at a time, and get various errors.  (1 /
> 1200 = 0.00083333...)
What errors?

>        #"wcs_resolution" "0.00083333 0.00083333"
Looks right
>        #"wcs_size" "70801 31201"
Looks right if you prefer to give size instead of resolution
>        "wcs_size" "31201 70801"
This is wrong.

> In case it's helpful, here's my LAYER definition (with probably extra things I've tried based on my searching and trial & error):

    LAYER
      NAME SRTM_3_arc-second_grid
      METADATA
        "wcs_label"           "SRTM - 3 arc-second grid"  ### required
        "wcs_rangeset_name"   "Range 1"  ### required to support DescribeCoverage request
        "wcs_rangeset_label"  "Lower 48" ### required to support DescribeCoverage request
        "ows_extent" "-125 24 -66 50"
        #"wcs_resolution" "0.00083333 0.00083333"    # 1/1200 for 3" spacing?
        #"wcs_size" "70801 31201"                    # Not sure of order
        "wcs_size" "31201 70801"
        "wcs_bandcount" "1"
        "wcs_native_format" "SRTMHGT"    # What should this be?  I can't find examples...

It is supposed to be the native format of the layer data, format that theoretically requires no processing, just selecting the right pixels. In practice the format that you users get by default with GetCoverage. It goes into DescribeCoverage about this way:
<wcs:nativeFormat>image/tiff</wcs:nativeFormat> 
and it is documented in the WCS 2.0.1 Core standard:
"4.7 Native Format
encoding format where, in a GetCoverage request, the range set values can be obtained unaltered"

        "wcs_srs" "EPSG:4326"
      END

      TYPE RASTER ### required
      STATUS ON
      TILEINDEX "../srtm_3_hgt/srtm_3_hgt-index.shp" # Path is relative to SHAPEPATH
      TILEITEM "Location"
      PROJECTION
        "init=epsg:4326"
      END
    END

> I feel like I must be close.  Thanks a lot for any further pointers,

-Jukka-

> carl