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 -- web:https://geographika.net & https://mapserverstudio.net mastodon: @[email protected] 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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