WTMS Layers: EPSG:2056 to EPSG:3857 scale difference

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Assigned to procras...@gmail.com by david...@gmail.com

Nicola Pfister

Mar 7, 2022, 4:23:55 AMMar 7
to GeoAdminCh API
Hello all,

We're using the Swisstopo WMTS services (https://api3.geo.admin.ch/services/sdiservices.html#supported-projections) and we've changed our applications spatial reference system to WGS84 from LV95 to be in line with modern standards and to be able to use the light base map in future. We've noticed a difference in scale of about 1:1.5 which results in completely different scale measures for the same view rectangles.

How can we explain this difference to our customers and is there a way to cirvumvent this? Is this maybe even an error in the WMTS definition?

Thanks for your support,
Regards, Nicola

Nicola Pfister

Mar 7, 2022, 5:05:36 AMMar 7
to GeoAdminCh API
We've found out some more details for this issue:

The request for a map tile in WGS84 (EPSG:3857) (https://wmts101.geo.admin.ch/1.0.0/ch.swisstopo.swissimage/default/current/3857/20/549057/367168.jpeg)
returns a tile of about 25m width in reality. Displayed on a screen at a width of 6.9 cm that results in a scale of about 1:360 but according to the WMTS Capabilities request, the ScaleDenominator value returned for this zoom level is 533:

The request for a map tile in LV95 (EPSG:2056) also delivers a tile that's about 25m in width in reality (https://wmts101.geo.admin.ch/1.0.0/ch.swisstopo.swissimage/default/current/2056/28/10173/3960.jpeg) does also result in a scale of about 1:360 but here the ScaleDenominator seems to be correct:

So we do seem to have an issue in that the WGS84 WMTS service returns an incorrect ScaleDenominator, leading to a different display at best and incorrect interpretation of displayed information at worst.

GeoAdminCh API

Mar 16, 2022, 12:27:19 PMMar 16
to GeoAdminCh API
Works as defined. The application ( in your case ARCGIS) reads the Tiles and assigns the correct scale. when using webmercator, scale varies within the map shown, depending on the extent of the map screen  https://glenbambrick.com/2016/02/03/web-mercator-fallacy/ 

usually, the scale shown in a webmercator map application is only valid for the center of the map
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