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I'm new to using GPS and using map projections. I've read quite a bit but I'm struggling to understand how to use Web Mercator projection.

I have lat/long coordinates from the nuScenes dataset. The nuScenes guidance says they use web Mercator projections. The lat/long is the bottom left-hand corner of a tile. The meters I want to add indicate where an autonomous vehicle begins its journey inside that tile.

I've added the meters (width and height) to get the lat/long location of the vehicle using the flat earth calculation (the meter distances are < 1.5km) however it seems that the error from doing this is quite large, ranging from 0.34km to 1.2km

I calculated the error by using the straight line distance from my new lat/long to identifiable landmarks seen in the video so there will be some errors which I accept but the current errors are larger than I was expecting.

From my very basic understanding, I can use the lat/long as they are but am I correct? How do I then plot the new lat/long on the map? Is this where I'm introducing the errors?

EDIT - From the comments I've had, I should explain better what I want to do.

I have a lat/long and I want to add metres to it to get a new lat/long. Do I have to concern myself with the web mercator projection? I don't want to project my new point on a map but the dataset says they use web mercator projection - will this have an impact on the lat/long or is the lat/long absolute and unaffected by projections?

Optimus_2016
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    Meters in web mercator are not real-worlds distances, but heavily distorted meters that make sense as coordinate-values only, not for measuring. So you should provide a bit more information what you intend to do - and what software you use. If it's only converting lat/lon values to WebMercator-coordinate values (in meters), you should use a transform-operation. – Babel Aug 23 '21 at 09:20
  • Thanks for replying. I want to add the metres to the initial lat/long to work out where the vehicle is. I don't want to map it, as such. I need the lat/long really just to know where in a city the vehicle is, the location is used as a trigger for another calculation. It's part of a larger program so the whole thing is written in Python, not using any of the specialist programs. – Optimus_2016 Aug 23 '21 at 09:31
  • Distance calculations in Web Mercator are useless, as your error measurements show. This appears to be an XY Problem, so you probably want to back up, and describe the problem Web Mercator was going to fix for you. – Vince Aug 23 '21 at 10:38
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    You must consider earths shape and the issue of projections to get any neaningful result. To get from lat/lon to meters means you have to reproject/transform from one CRS to another and then be sure to get the right meter-values (ellipsoidal, not planar measurement). Whereas all of this is doable if you know the underlaying basics of maths and spherical geometry, it is probably still more convenient to rely on existing Python packages or specialized GIS software if you don't want to re-invent the wheel and avoid errors. – Babel Aug 23 '21 at 10:47
  • Try pyproj4 https://github.com/pyproj4/pyproj as recommended here: https://gis.stackexchange.com/a/78944/88814 – Babel Aug 23 '21 at 11:45

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