Map scale mismatch with EPSG 3857

[samlaminilepre]

Steps to reproduce

1. Set coordinate reference system with epsg code 3857
2. Import .osm file
3. Measure the distance beetween two features easy to recognize
4. Compare the distance with the one measure on googlemaps or other

Actual behaviour

The distance on OOM is around 40% bigger

Expected behaviour

It should be the same distance

Configuration

Mapper Version: 0.8.4 Operating System: Windows

PS. Georeferencing seems to work fine even with the scale problem: if you import other georeferenced templates they match without problem and live gps while mapping on android work fine too. I'm not an expert but it looks to be only a scale problem... PPS. The only way to have the right scale while importing .osm file was to use "local" as map georeferencing option. But then live gps won't work... Thank everybody

[dg0yt]

While I can reproduce the issue, it probably won't be fixed in Mapper. I would rather suggest that you choose another coordinate reference system (CRS), e.g. UTM.

Mapper assumes that the projected CRS uses coordinates in meters. This serves us well in most cases. However, EPSG 3857, aka Web Mercator, is optimized for web applications. While it seems to use meters, an "EPSG 3857 coordinate meter" is a quite different length from the regular meter on the earth's surface when you are far away from the equator. It is rather the projection of a meter of the equators circumfence to the circumfence at you coordinates' latitude. That's why you end up with more "EPSG 3857 coordinate meters" than you would expect.

All projections are due to some error when you put the world on a flat map. The "Grid scale factor" field is an adjustment which may compensate or at least mitigate the issue. But Mapper doesn't provide the value for you.

Disclaimer: I'm not a professional in cartography, so my explanation might be inaccurate or even wrong. Find a related question and answer here: https://gis.stackexchange.com/questions/242545/how-can-epsg3857-be-in-meters or stay tuned for comments from some of our users which do know much better indeed.

[jmacura]

@dg0yt 's answer is correct, but to provide a bit more context: every map projection does distort the lengths, even the ones from category "equidistant" (they only preserve the length in one direction). This is one of the reasons why there is so many map projections. Each of them can have a different use case, each of them may be intended for different part of the world. For example Web Mercator (EPSG 3857) is suitable for length measuring only around the equator (and by "around" I mean around the world, along the equator).

There definitely exists some projection which will suit better your area of interest. A good starting point might be to choose one of the UTM zones, depending on your latitude and longitude. Or do you have any special reason why to work in Web Mercator?

[samlaminilepre]

Thanks both for your answers. Now it seems to work!! I know something about projection but i was expecting that OOM was able to scale it like google maps do even using web mercator, I was wrong and problably is a complicated to add this feature in the program and it's not really necessary so... I thought to work with 3785 because in my mind was logic to work in the same projection as my import data. Maybe could be a good idea to add this info in the online manual for starting a new map, so other won't do the same mistake as me!

[jmacura]

@samlaminilepre +1 for adding it to the manual. It is probably not that straight-forward for the most users.

What Google and other map providers does under the hood, is actually recalculating the distance between two points on a sphere (or ellipsoid maybe) independently on the projection.

[pkturner]

Perhaps Mapper could handle this. See pull request #1210, Automatic Grid Scale Factor.