[mapserver-dev] Scales and raster maps

[Frank Broniewski]

Hello all,

yesterday I posted a thread on the Mapproxy list concerning map scales 
and the DPI setting [1]:

> TL;DR
> The scale isn't the scale and a screen doesn't have dots so it doesn't have DPI. Don't do scale dependant calculations on fictional units.

This is something that keeps my mind busy for quite some time. We're 
assuming a certain DPI value, which doesn't make any sense for screen 
mediums at all, and this DPI value isn't probably correct for most of 
the time. Please read my post on the mapproxy list for a more verbose 
discussion why raster map scales -stink- are IMO not good.

Since this DPI value is used for scale computations and scale is used 
for many vital parts in a map's composition I'm having a sore tummy (is 
that a correct phrase?) all the time. You define your layout based on 
scale and you define layer visibility on scale and probably even more 
settings.

Issues arise also when tuning your client  vs. server application and 
both have a different understanding of scale and / or this DPI setting.

What I'd like to have and propose hereby is a scale independent ratio 
like most Javascript clients and also Mapproxy have which is the 
resolution. This is a ground unit per pixel ratio and is something that 
is valid throughout all screen pixel densities.

Would this be possible to implement along the DPI/scale setting? I think 
this would be handy to have and would be also more future proof than the 
DPI/scale setting. Since this measurement is not based on an arbitrary 
estimated value but is based on real output dimensions that is the 
pixel. Well, most of the time I mean ;-)


Many thanks for reading,

Frank


[1] 
http://osgeo-org.1560.x6.nabble.com/MapProxy-Seeding-in-a-certain-scale-tp5071082p5071464.html

-- 
Frank BRONIEWSKI

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