[mapserver-users] WMS: raster's x and y pixel dimension
Gerald Creager
gerry.creager at tamu.edu
Tue Dec 1 15:09:43 PST 2009
And, as usual, I was too fast for my own good.
Your last sentence summarized the abstracts of the Open Geospatial
Consortium for WMS, WFS and WCS. However, it begs the question of
defining the terms. A knowledge of teh definitions is necessary to
understand what you want to access and what you're getting. Mistakes
are common where one thinks they understand a "feature" but then get
something back that didn't match their understanding. I could
substitute 'map' or 'coverage' for feature above.
And then, there's the metadata. Metadata and a coherent vocabulary and
ontology, often formated as a registered schema (GML in this case)
provides the description of the data for further use. When one issues a
GetCapabilities request to a Web [Map|Feature|Coverage] server one gets
a GML response back with all the pertinent metadata available about the
requested object that's on that server. From that, one may craft a
request for the data and data-type desired. While you're working with
that object, you will need to retain the metadata.
Note that in my earlier post, I indicated use of a 3-band geotiff of
landsat data. This represents a processed image, as you've restricted
the dataset to those three bands of data. The netCDF example was an
unprocessed (potentially) example (Hmmm. Maybe I should add some
cgi-hooks for NCAR's NCL package for netcdf subsetting and processing?).
The "WxS" packages, as I've taken to calling them, are powerful tools
for their specific tasks. They can be used out-of-context, too, but
that tends to confuse others who aren't familiar with how you were
thinking if you ask them to do strange things.
Your approach to this does not appear incorrect to me, but doesn't take
advantage of the full capabilities of the services.
gerry
Rahkonen Jukka wrote:
> Hi,
>
> Thank you for clarifying my quick answer. I am looking WCS from a narrow viewport because I use it mostly for delivering aerial images in original resolution and quality. WCS does give more. Delivering raw data is not the only alternative with WCS but it offers also some limited processing options like resampling to different resolution, reprojecting and a set of user selectable outputformats.
> When it comes to spatial relevance, for me the image captured from WMS does not have much spatial relevance after it has arrived to the computer if the request used for ordering it from the server is not known. GIS program who made the request does remember it but save it on a disk, or capture it with a web browser and it is just an image. Of course the situation is similar if you take an image from WCS server in png or jpeg format which do not carry embedded metadata. But this is not black and white either, Mapserver WMS can also create georeferenced geotiff or ecw files, even most WMS clients do not know what do do with them. Is it too simple to say that WMS was planned to be used for showing a map, WCS for downloading coverages and WFS for querying and downloading features?
>
> -Jukka-
>
> Gerald Creager wrote:
>
> Er... not quite.
>
> WMS is designed to provide a map for, say, a baselayer or background
> with spatial relevance for GIS work. A picture on a screen is similar,
> but a WMS result has spatial relevance.
>
> Web Coverage Service delivers a product with spatial relevance and data
> embedded within the pixels, voxels, etc of the coverage. You're not too
> far off, but I believe you're attempting to oversimplify this.
>
> A WFS result will provide a feature (line, polygon, point, polyline,
> etc.) with all of its attributes.
>
> In a coverage, e.g., a geotiff of a 3-band LandSAT image, you would have
> information on which bands were provided for the RGB elements, and the
> pixel values for each. The coverage is georegistered so that the
> spatial relevance of the "image" is preserved. Similarly, a netcdf with
> significantly more values could be passed as a coverage and utilized,
> not limited to simply 3 "bands" of "color" (data).
>
> gerry
>
> Rahkonen Jukka wrote:
>> Hi,
>>
>> I would say that the main difference is that WMS is meant for showing a map on a computer screen, while WCS is made for letting users to download data so they can utilise it locally or process further. Real life is not so black and white. I promise you will find lots of good reading about WMS and WCS easily. Do not forget to read a little bit about WFS as well, service for downloading vector data.
>>
>> -Jukka Rahkonen-
>>
>>
>> fla83tn wrote:
>>
>> Lähetetty: ti 1.12.2009 17:31
>> Vastaanottaja: Rahkonen Jukka; mapserver-users at lists.osgeo.org
>> Aihe: Re: Re: [mapserver-users] WMS: raster's x and y pixel dimension
>>
>> Thank you Jukka for the quick reply. Now I'm able to correctly generate z-
>> dimension from DEM layer.
>> However, since I'm new to GIS, I'm not able to
>> understand the differences between WCS and WMS..could
>> you briefly summarize
>> the goal of the two approach?
>> And, as last question , should I set the resX and
>> resY parameters from client or could I embed them directly into the map file?
>>
>>
>> Best regards,
>> Flavio
>>
>>> Hi,
>>>
>>> Width, height and BBOX are all compulsory
>> parameters in WMS GetMap
>>> request and Mapserver is sending an image according
>> to the request. If
>>> you want to be able to control the pixel size easily,
>> without making WMS
>>> client to balance the BBOX and image size in pixels,
>> consider using WCS.
>>> One way to make a WCS request is
>>>
>>> http://server.fi/cgi-
>> bin/mapserver_wcs?SERVICE=WCS&VERSION=1.0.0&REQUEST
>>
>>> =GetCoverage&COVERAGE=DEM&FORMAT=image/tiff&CRS=EPSG:3067&ResX=1.0&ResY=
>>> 1.
>> 0&BBOX=533652,6973434,534658,6974219
>>> You can see parameters ResX and ResY
>> which are used for controlling the
>>> pixel size.
>>>
>>> -Jukka Rahkonen-
>>>
>>> fla83tn
>> wrote:
>>>> Hi to all,
>>>> I've a problem with the loading of a raster
>> image via WMS (1.3.0).
>>>> To load the layers of my project I use gvSIG. If
>> I load my
>>>> digital elevation model (DEM) directly from filesystem (which
>>>>
>> is a HFA/Erdas Image file - .img),
>>>> x and y pixel dimension are fixed to
>> 2.5 and -2.5 meters
>>>> respectively, and fixed are also the width and height
>> attributes.
>>>> If I load the layer from
>>>> Mapserver's WMS instead, x and y
>> pixel dimension change as I
>>>> zoom in or out (i.
>>>> e. changing the width
>>>>
>> and height parameters of the getMap request).
>>>> Is there
>>>> the possibility
>> to tell the wms client that x and y pixel
>>>> dimension are constant and should
>> never be calculated?
>>>> Here's my layer definition and the
>>>> tiff
>> outputformat definition:
>>>> OUTPUTFORMAT
>>>> NAME GTiff
>>>> DRIVER
>> "GDAL/GTiff"
>>>> MIMETYPE "image/tiff"
>>>> IMAGEMODE FLOAT32
>>>>
>> EXTENSION "tif"
>>>> END
>>>>
>>>> LAYER
>>>> NAME
>>>> "my_dem"
>>>> EXTENT 669913.75
>> 5143998.75 678313.75 5150701.25
>>>>
>>>> METADATA
>>>>
>>>> "wms_title"
>> "my_dem" ##required
>>>> "wms_extent" "669913.75 5143998.75
>>>> 678313.75
>> 5150701.25"
>>>> "gml_include_items" "all"
>>>> END
>>>> TYPE RASTER
>>>>
>>>>
>> STATUS ON
>>>> PROJECTION
>>>> "init=epsg:25832"
>>>> END
>>>>
>>>>
>> DATA "my_dem.tiff"
>>>> PROCESSING "NODATA=-9999"
>>>> PROCESSING
>> "SCALE=227.23,1741.15"
>>>> PROCESSING
>>>> "SCALE_BUCKETS=2000"
>>>> PROCESSING
>> "BANDS=1"
>>>>
>>>> DEBUG on
>>>> DUMP true
>>>>
>>>> TEMPLATE "void" # enable
>> queryable attribute on the layer END
>>>> Any help is
>>>> appreciated,
>>>>
>> Flavio
>>>> _______________________________________________
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>> mailing list
>>>> mapserver-users at lists.osgeo.org
>>>> http://lists.osgeo.
>> org/mailman/listinfo/mapserver-users
>>
>>
>>
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>
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