[mapserver-dev] Scales and raster maps

[Frank Broniewski]

Hi Thomas,

Am 2013-08-09 09:11, schrieb 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...

my point is probably a bit academic and I really don't want to replace 
the (existing) scale systems in our web mapping applications. Problem is 
that the scale that you are using for the determination of what you want 
to display is totally wrong. When you deliver a map with a certain pixel 
resolution (e.g. 800x800 pixels) it's scale varies greatly depending on 
the output device. OGC defines a pixel with 0.28^2 mm, the Iphone has a 
pixel size of 0.0779^2 mm. So the "real" dimension of map is totally 
different depending on the device and therefore the scale.
It's maybe the word scale that bothers me here, because it is not *the* 
map scale we refer to when we talk about maps, but it is a ratio of real 
world dimensions against a internal setting.

>
> snip ...


> 1. Openlayers allows you to work with scale *and* resolution, which is
> even more confusing.
I know, it wasn't probably the best example taken but I think this is 
something to consider. A two way approach ...

> 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....)

Again, your scale is based on wrong assumptions because you do not know 
the output mediums resolution at all. Leaving the DPI/PPI issues aside, 
you simply cannot know what dimension your map will span on the output 
medium. Scales need to be precise. If a user requests a map at a scale 
of e.g. 1:20.000 the scale needs to be the same across all output 
mediums. With paper maps (and PDF) it's easy because you know the 
dimensions. But with raster maps it's virtually impossible to know.
And I think it is more precise, better easier, to define a style for a 
pixel with 5mx5m resolution than for a somewhat vague 1:20.000 scale 
(numbers are not related).

Other problems arise when a client software doesn't have the same 
assumption about the DPI setting like Mapserver has (or you have defined 
in the Mapfile). I found this post on the QGIS hub [1] where the map 
layout is broken because QGIS does have another view on the clients DPI 
as the OGC has. Because then 1:20.000 isn't 1:20.000 anymore and the 
wrong map gets delivered to the client.

>>
>> 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 ?

What I would like to -have- see as an addition is another way of 
defining my cartography rules. Something like the resolutions from 
Mapproxy (to leave out OpenLayers for this time, even if Mapproxy isn't 
a cartography tool by itself) . I'd find it really helpful to make my 
styles dependent on pixel to ground units ratio than scales. May it be 
feet per pixel, meters, km or whatever. To speak in Mapfile terms I'd 
like to have an addition to the MIN- and MAXSCALEDENOMINATOR parameter. 
Something like a MIN- and MAXRESOLUTIONDENOMITATOR. The resolution 
itself is always easily calculable because you know the map's extent and 
the pixel output size. Having this mechanism available would decouple 
Mapservers output from any assumptions about the client.

Just to repeat it: I don't think it is feasible to remove the scale 
based mechanism from Mapserver, what I think would be helpful is a more 
device neutral styling determination mechanism as an addition.


thanks,

Frank


[1] http://hub.qgis.org/issues/6430

>
> 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
>>
>> METRICO s.à r.l.
>> géomètres
>> technologies d'information géographique
>> rue des Romains 36
>> L-5433 NIEDERDONVEN
>>
>> tél.: +352 26 74 94 - 28
>> fax.: +352 26 74 94 99
>> http://www.metrico.lu
>> _______________________________________________
>> mapserver-dev mailing list
>> mapserver-dev at lists.osgeo.org
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>
>


-- 
Frank BRONIEWSKI

METRICO s.à r.l.
géomètres
technologies d'information géographique
rue des Romains 36
L-5433 NIEDERDONVEN

tél.: +352 26 74 94 - 28
fax.: +352 26 74 94 99
http://www.metrico.lu