WMS Capabilities via Rest: MinScaleDenominator MaxScaleDenominator values

Question

Hi When I add a ArcGIS WMS service endpoint to a REST service endpoint that have Min/Max scale values set, the scale limits are available as MinScaleDenominator & MaxScaleDenominator values in the GetCapabilities response - with slightly different values. For example, 1:600000 in the REST service becomes 1:566964.285714 in the WMS service endpoint. If I add the same WMS into ArcMap, ArcMap has the intelligence to know the 'real' scale and convert it back to 1:599999. It seems the projection has an effect: My REST service endpoint is in spatial reference 2157, while the WMS GetCapabilities response list the following options in this order: CRS:84 EPSG:4326 EPSG:2157 Now, if i have a client that should do the same calculation to get the 'real' scale, the following questions pop up:* Is the order of the CRS's important in the WMS GetCapabilities response? * How does ArcMap know that the actual spatial reference is 2157? * What is the calculation ArcMap do calculate 599999? Any response would be appreciated, regards, gert

YingqiTang · Answer

Hi Gert, The difference between 1:600000 and 1:566964.285714 is not related to the coordinate reference system, but caused by the DPI definition in WMS specification. In WMS spec it defines that one pixel is 0.28mm in length which leads to a DPI around 90.7 (not exactly 90.7 though, it has more digits after). The scales values in REST assumes 96 DPI so both WMS server (writes out Min/MaxScaleDenominator) and ArcMap has the logic to convert them back and forth. Thanks, YingqiHi When I add a ArcGIS WMS service endpoint to a REST service endpoint that have Min/Max scale values set, the scale limits are available as MinScaleDenominator & MaxScaleDenominator values in the GetCapabilities response - with slightly different values. For example, 1:600000 in the REST service becomes 1:566964.285714 in the WMS service endpoint. If I add the same WMS into ArcMap, ArcMap has the intelligence to know the 'real' scale and convert it back to 1:599999. It seems the projection has an effect: My REST service endpoint is in spatial reference 2157, while the WMS GetCapabilities response list the following options in this order: CRS:84 EPSG:4326 EPSG:2157 Now, if i have a client that should do the same calculation to get the 'real' scale, the following questions pop up:* Is the order of the CRS's important in the WMS GetCapabilities response? * How does ArcMap know that the actual spatial reference is 2157? * What is the calculation ArcMap do calculate 599999? Any response would be appreciated, regards, gert