[MapServer-dev] Call for review: RFC-142: Scalebar Measurement Modes

[Seth G]

Hi Tamas,

OK noted re-backwards compatibility and stability between requests - seems safest to make this a new keyword / option.

If the performance cost is negligible then a user can pick whichever approach is most suitable without worrying about performance (which is good!).

Middle-of the map seems the cleanest approach.

Thanks for addressing my questions (and a pre-emptive +1 from me),

Seth

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On Sun, May 17, 2026, at 1:48 PM, Tamas Szekeres wrote:
> Hi Seth,
> 
> Thank you for the feedback, see my comments inline:
> 
>> 
>> I'd agree with Even's point that using the "geodesic" approach for any projected map would make sense - or is there a situation where you'd want a Cartesiam scalebar drawn? Maybe to have consistency between requests?
>> 
> 
> Geodesic measurement is likely the better default for most projected maps intended to show real-world ground distance, especially EPSG:3857. However, preserving cartesian measurement gives users a deterministic legacy-compatible option where the scalebar represents map units directly and remains stable across requests. This is useful for existing applications, tests, cached map output, or workflows where projected coordinates rather than ellipsoidal ground distance are the intended reference.
>  
>> 
>> I guess the new approach could slow down the generation of the scalebar, but as embedded scalebars are part of full Map requests (and not WFS etc.), it wouldn't affect most uses of MapServer. 
>> 
> 
> The added cost should be negligible for normal embedded scalebars. A geodesic scalebar currently adds two coordinate transformations and one proj_geod() call per scalebar fitting iteration, usually only one or a few iterations per rendered map. This is tiny compared with a full map render, although we can mention that pathological scalebar-only or repeated-render benchmarks may show a measurable difference.
>  
>> 
>> Using the middle of the map for the scale seems the simplest option. If a user renders a world-wide Web Mercator map, and the scalebar is in the top-left of the map, then using pixels at this location would create a scalebar based on pixels with high latitudes and a 100 KM scalebar could represent 50 KM on the ground. Using the map center would produce a scalebar, only accurate at the equator, but at least consistent with the current implementation.
>> 
> 
> I agree with this standpoint and adjust the RFC/implementation accordingly: geodesic mode should sample at the map center, while POSITION/OFFSET should remain purely visual layout controls.
> 
> Best regards,
> 
> Tamas
> 
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