[Community] Re: FW: Re: [PrimaGIS and Cartographic Objects for Zope] #99: Style Validation Check for Combined Min/Max Scales wrong
kai.hanninen at mbconcert.fi
Mon Jul 3 14:37:50 EEST 2006
Sean Gillies wrote:
> On Jul 2, 2006, at 5:26 AM, Ludwig Max Brinckmann wrote:
>> >#99: Style Validation Check for Combined Min/Max Scales wrong
>> > Reporter: ludwigbrinckmann at hotmail.com | Owner: dokai
>> > Type: defect | Status: assigned
>> > Priority: minor | Milestone: PrimaGIS-0.6
>> > Component: PrimaGIS | Version: SVN
>> >Resolution: | Keywords: Styling
>> >Comment (by dokai):
>> > Revision r742 has a fix for this.
>> > Note that the validation of the min/max scale values has not been
>> > though. If we have two scales, say 1:20000 (the smaller scale) and
>> > (the larger scale), then to have features rendered only within these
>> > scales one would set
>> > min scale = 1:20000
>> > max scale = 1:10000
>> > The problem was that PCL deals with scale denominators where the
>> > of min and max are the opposite.
>> I must say that I find this notation confusing, although technically
>> of course you are right as we are dealing in fractions. I adapted my
>> script to create my map to the new notation and it took me a few
>> attempts to get it all right. I like to think it as a to-from range,
>> eg. from 1:10000 to 1:20000, where min is 1:10000 and max is 1:20000.
>> If you are going to the edit pages, their layout suggests this: we
>> like to have the smaller numbers (min) on the left, the larger numbers
>> (max) on the right. Only if I read the columns right to left, they
>> make sense to me.
>> As I said I cannot argue against the logic on a mathematical level and
>> maybe it is just me. I would argue though that PCL and Primagis should
>> come to an agreement on the meaning of this.
>> I had hoped that this would also solve the problem I reported a few
>> days ago with rules (on the way to primagis 0.6). It doesn't, but I
>> think the problem might be related.
> Yes, it's confusing. I suggest that when possible we try to have large
> scales and increasing magnification to the left and small scales
> (decreasing magnification) to the right. A map scale slider should be
> presented like
> zoom in zoom out
> Ludwig, if we did the same for ranges
> [zoomed_in, zoomed_out]
> would that help?
> I chose to have PCL deal in terms of denominators mainly because of the
> SLD specification, but also because there is a tradition among GIS
> practitioners of referring to the denominator when speaking about map
For me the issue of map scales is also a source of confusion sometimes
particularly as I don't have a strong GIS backround. I really don't have
a strong oppinion here and would gladly have PrimaGIS present the matter
of configuring the scale thresholds in the most simple and easy-to-grasp
way to the user (or Plone administrator).
The biggest problems concerning map scales (at least from my
perspective) is the concept of ordering. So is the scale of 1:1000 less
or more than 1:20000? I believe the official view is the same as the
mathematical view where 1:1000 (= 0.0001) > 1:20000 (= 0.00005).
For the average user the 1:1000 (with the number 1000 in it) can easily
seem less than 1:20000 (with the number 20000 in it) and that would lead
one to assert that 1:1000 < 1:20000. I guess this is the reason for the
tradition of using just the scale denominators, as Sean mentioned above.
This brings us to the next problem :) PCL uses the terms "min scale
denominator" and "max scale denominator" to refer to the thresholds, but
their meaning (at least to me) is somewhat ambigous and I believe this
was what originally confused my and introduced the bug in PrimaGIS.
"min scale denominator" can (imo) be interpreted either as:
1) The minimum of scale denominators
>>> min_of_scale_denominators("1:1000", "1:20000")
2) The denominator of the minimum scale
>>> denominator_of_min_scale("1:1000", "1:20000")
Using just the denominators is fine by me as the fractional
representation is mostly redundant. I do believe that the terminology
should be made clearer though.
UI wise, the reason why I chose to represent the the min/max scale
thresholds in the "1:XXXXX" format in the PrimaGIS UI is that this is
what you see on paper maps. I felt that it would make it easier for
users to manage them if they were similar to maps they might have had
experience before. (It seems that I was wrong).
To me, PrimaGIS should currently works "right" when using the semantics
of mathematical ordering, (e.g 1:1000 > 1:20000) and users would use
min scale threshold = 1:20000
max scale threshold = 1:1000
Until we can agree on how to interpret and present the map scale
thresholds to the users I don't see there's much we can do to "fix" it :)
Kai Hänninen +358-50-558-7935
Software engineer www.mbconcert.fi
MB Concert Ky kai.hanninen at mbconcert.fi
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