Question

Reprojector not uniform


Hi!

In a project, I want to plot the bounding points of an area onto a map. To do that I am mainly using the Featurereader to grab the map from a WMS using said bounding points (as a polygon). In order to do this to grab the map, I need to change the coordinate system. Thats why I'm using a Reprojector to convert from the national coordinate system SWEREF991800 to EPSG:4326, which is the proper one for the WMS.

The operation kind of works, I manage to grab the map including my bounding points, and I can confirm it is the correct area. However, the reprojected points and the WSM appear scaled in the X-axis, instead of being almost a square. Please see the attached pictures. Why am I seeing this, is it a Reprojection problem?


6 replies

Userlevel 5
Badge +29

I would say its correct and its due to distortion of the coordinate system. You're going from a local specific coordinate system (at reletively high latitudes) to a global coordinate system. It would be expected to see distortion

I would say its correct and its due to distortion of the coordinate system. You're going from a local specific coordinate system (at reletively high latitudes) to a global coordinate system. It would be expected to see distortion

Alright, thank you.

I'm assuming this can't be corrected somehow in the reprojector or elsewhere?

Userlevel 5
Badge +29

Alright, thank you.

I'm assuming this can't be corrected somehow in the reprojector or elsewhere?

If your WMS service supports EPSG:3857 (Web Mercator) that should give a less distorted result.

Theres nothing you can change in the reprojector to fix it. Think of when you peel an orange and then try to flatten the skin, the edge rip and distort. Thats basically whats happening with your buildings

Alright, thank you.

I'm assuming this can't be corrected somehow in the reprojector or elsewhere?

Thank you. I see, makes sense. What I did to solve the problem for now was to calculate the ratio between the long x and y side respectively on the original coord system and scaled the reprojection to match the ratio.
Userlevel 5
Badge +29

Alright, thank you.

I'm assuming this can't be corrected somehow in the reprojector or elsewhere?

By scalling the data, whilst it may now 'look' correct, it is actually wrong and you have changed the physical positioning and dimensions of that data. Different coordinate systems distort data differently. And compromises need to be made accordingly. That's why there are so many different coordinate systems available to use.

 

I'd suggest you have a look here: https://safe.my.trailhead.com/en/content/safe/modules/learn-spatial-data-concepts

 

Specifically "Distinguish Between Geographic and Projected Coordinate Systems"

 

Alright, thank you.

I'm assuming this can't be corrected somehow in the reprojector or elsewhere?

Ok, I'll check that out. Actually I'm just rescaling the "print" of the wms and data for a pdf and not using the data further than that.

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