[mapserver-dev] Scales and raster maps

[thomas bonfort]

Frank,
While I agree with you that the concept of "scale" is not suited for
digital maps, I'm not seeing it go away as it is the the only metric
that gives you a tangible sense of "what should I be displaying when
I'm zoomed in at a given level". more inline...

On 9 August 2013 08:19, Frank Broniewski <brfr at metrico.lu> wrote:
> 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.
1. Openlayers allows you to work with scale *and* resolution, which is
even more confusing.
2. While pixels per ground unit is great for tiling because "ground
unit" is strictly defined by your grid definition, this does not fit
with a mapserver where you want to define scale-based ruled rendering,
but you have no idea what the ground unit will be (i.e. depending on
the request, it can be meters, degrees, feet, miles, nautical
miles....)
>
> 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 ;-)

What are you proposing concretely ?

regards,
thomas

>
>
> 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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