[MapProxy] zoom levels and resolution

[A.Pirard.Papou]

On 2012-10-01 14:07, Oliver Tonnhofer wrote :
> On 01.10.2012, at 05:45, André Pirard wrote:
>> I was using a cache that, without any res configuration created folders 16, 17, 18 and 19 which I relate to zoom levels.
>> To follow your method, I used
>>    debug_source:
>>       type: debug
>> This painted the following first 2 numbers, roughly at the indicated meters/JOSM scale (2.5 screen cm).
>> I suggest you write them to standard output (instead or in addition).
>> I have absolutely no idea what calculation produces those numbers.
>> I assumed they correspond to the former 19 and 18, I continued multiplying by 2 and I got:
>> 19:   0.29858214: 4, 11,  22,
>> 18:   0.59716428: 63
>> 17:   1.19432856
>> 16:   2.38865712
>> 15:   4.77731424
>> 14:   9.55462848
>> 13: 19.10925696
>> 12: 38.21851392
>> 11: 76.43702784
>>
>> I chose to use:
>> res: [76.43702784, 19.10925696, 4.77731424, 1.19432856, 0.29858214]
>>
>> Now I surprised to find directories 4 and 3 instead of 19 and 17.
>> Why?  It would have been simple.
> Because you told MapProxy to do so :)
> You configured a grid with 5 levels, from level 0 at resolution 76.43702784 to level 4 at resolution 0.29858214.
>
>
>> Q5: is there anything to read on the Web that would help answering not only Q1-Q4 but also Q6-Q∞? :-)
> The question is: What are you trying to do and what did not work with your first configuration (without res)?
>
>
> Regards,
> Oliver
The practical goal is to remove some of the zoom levels directories from 
the cache to minimize server requests and conserve disk space (as the 
doc explains or implies, I think).
In my case, the highest resolution is kept and one or two lower levels 
to navigate.
It's very tedious to 1) compute resolutions with debug 2) have to rename 
the directories to new, unpredicted numbers and keep the previous 
contents without goofing 3) experiment and loop repeating (2) with yet 
different directory names each time

Suggestion:

Wouldn't it be possible to specify sth like    res: [16:2.38865712, 
19:0.29858214]
to indicate to use the existing directories named  16 and 19 with their 
contents and with the indicated resolution.  If other names are 
required, symbolic links can be automatically created.  The user 
shouldn't know the new numbers except for possibly erasing them.

Better yet, to specify  res: [16:, 19:] meaning the same except that the 
original, default resolutions are kept.  In other words, the user just 
selects the original, default zoom levels that she/he wants to keep and 
she/he can change her/his (or even his/her ;-) mind later.

In the meantime, without that comfort, it would be interesting
1) to know how the default resolutions are computed: can we get them 
from a server query instead of the debug method?
2) said unselfishly: to learn what you said from the doc but in terms of 
directory names: level 0 to 5 = what filenames if you don't succeed 
creating more than 3 and 4?
3) to understand how an application uses the defined levels to force 
filling the right ones;
in JOSM I see a) change resolution b) zoom to native resolution but I 
don't relate that to file numbers.

Any how it works is appreciated because it implies 20+ howtos;  the 
opposite is not true.

Thanks,
Cheers,

André.


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