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NeXtMidas User's Guide
Third-Party Compatibility
WMS and GeoWebCache
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NeXtMidas support for WMS and GeoWebCache
NeXtMidas has support for downloading maps from a Web Map Service.
A GeoWebCache acts as a proxy between a GeoServer and a map client.
In order to get cached results from a GeoWebCache, extra guildlines must be followed.
Full GeoWebCache guidelines
- TILED=true is included in the request
- Only one layer can be referenced in the getMap request
- Image requested same height/width saved in layer configuration
- The requested CRS matches one of the available tile layer gridsets
- The image requested lines up with the existing grid bounds
- Only supported Parameter filters are included in the request
Guidelines for NeXtMidas interaction with a standard GeoWebCache
- Include GWC=TRUE as one of the WMS Keyword Parameters
- This will force TILED=true to be included in the requests
- NeXtMidas will prefer the CRS EPSG:4326, which is one of the two standard CRSs supported by GeoWebCache
- The tile requests will be 256x256 matching the EPSG:4326 height/width requirements
- The tile image bounds will align with the EPSG:4326 grid bounds
- BGCOLOR will NOT be included in the getMap request, as it is a parameter filter and not supported by default
- LAYERS= should reference only one layer or TRANSITIONS= should be set, so only one layer is requested at a time
Sample Standard Configuration GeoWebCache compliant WMS
- A NeXtMidas command such as:
nM> plot {LT=WMS,URL=mapURL,Layers="shapePolitical,Relief,cadrg,imint",&
  
TRANSITIONS="500,1.5e3,1e8",GWC=true,NumRetriesIfBGColor=4}
- This results in tiled requests similiar to this:
url/maps/geoserver/MapData/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&CRS=EPSG:4326&BBOX=42.69287109375,-71.47705078125,42.71484375,-71.455078125&WIDTH=256&HEIGHT=256&LAYERS=imint&STYLES=&FORMAT=image/png&EXCEPTIONS=INIMAGE&TRANSPARENT=FALSE&TILED=true
WMS Keyword Parameters
- LT=WMS
- Specifies layer type as LayerWMS
- URL=
- Specifies the URL of the WMS server
- LAYERS=
- TRANSITIONS=
- A pixel-to-degrees ratio at which NeXtMidas should switch to the next layer
- Note: enabling the onAxis readout of WMS_level can assist in choosing these
transition values. READOUTEXT=+Show|+WMS_level
- GWC=true
- GeoWebCache friendly behavior enabled, including TILED=true
- NumRetriesIfBGColor=
- Retries for a good tile if an errorless background only color tile is received.
- IGNOREHREF=true
- Avoids having to wait for a timeout if a WMS server is improperly configured
and returning an internal HREF in GetCapabilities
- ByNameCRSs=
- Allows the preferred CRS(s) to be directly specified by comma separated name(s).
- CRSs are arbitrated based on the standard GetCapabilities request.
- If the map server is erroneously returning more CRSs than it actually supports,
this can cause an unsupported CRS to be chosen.
- The default CRS in NeXtMidas 4.1.0+ is EPSG:4326, but earlier versions of NeXtMidas have CRS:84 as the default.
- RequestedVersion=
- WMS 1.3.0 will be preferred by default. Specifying "1.1.1" will change the preference to 1.1.1.
- RequestedVersion must come BEFORE the URL specification.
- This is useful if the WMS server has lat/lon ordering incorrect for WMS 1.3.0.
- ConnectTimeout=
- To override the default network connection open timeout
- ConnectTimeout must come BEFORE the URL specification.
- ReadTimeout=
- To override the default network connection read timeout
- ReadTimeout must come BEFORE the URL specification.
- AutoConnect=false
- Disable the automatic connection to the Map Server while setting the URL
- AutoConnect=false must come BEFORE the URL specification.
- BRIGHTNESS=
- CONTRAST=
- DOWNLOADTHREADS=
- Set the number of download threads.
- NeXtMidas 4.0.0+ has the default being 32.
- Older versions of NeXtMidas had the default as 4.
- CACHESIZE=
- Set the number of map images cached.
- NeXtMidas 4.0.0+ the default tiled cache was updated be intelligent and based on available JVM memory, with a maximum of 1024.
- Older versions of NeXtMidas had the default tiled cache size as 16.
- The default cache size when not doing tiled requests remains 4.
Custom GeoWebCache configurations are not currently supported
- Custom GeoWebCache configurations with different height/width or grid bounds are not currently supported
- In order to support this, NeXtMidas would need to make a separate WMS 1.1.1 GetCapabilities
request and act on that information
url/maps/geoserver/gwc/service/wms?SERVICE=WMS&VERSION=1.1.1&REQUEST=GetCapabilities&TILED=true
- The VendorSpecificCapabilities TileSet section of the response contains
- SRS specific specifications
- BoundingBox
- Resolutions
- Width
- Height
- Format
- Layers
- Styles
- Layer specific specifications
- Name
- Title
- Abstract
- SRS
- LatLonBoundingBox minx= miny= maxx= maxy=
- BoundingBox SRS= minx= miny= maxx= maxy=
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