[Tiling] FOSS4G BOF?

[Steven M. Ottens]

Op 8/22/2011 4:16 PM, Oliver Tonnhofer schreef:
> Hi tilers,
>
> there are lots of things to discuss in the tiling BOF and I want to attend the Python BOF as well. Maybe we can find some time at the code sprint?
Codesprint is fine also for me (although I prefer to do some actual 
codesprinting there as well)
> On 20.08.2011, at 19:19, Gabriel Roldan wrote:
>> But the problem we face is that (to the extent of my knowledge) there
>> are no pre-defined "grid sets" for EPSG coordinate reference systems
>> so it's hard to configure "tiling schemes" or "gridsets" for CRS's
>> other than, say, 4326 and 900913. Not to talk about making them
>> "inter-operable" like in making tiled layers coming from different
>> tile servers match.
>> Now, the EPSG database contains each (or almost each) CRS's area of
>> validity with unit of measurement information and all. But also I
>> guess most servers don't need/want to rely on the EPSG database as
>> it's just easier for them to work on pure cartesian space with some
>> independently of most CRS details but perhaps bounds/tile origin
>> coordinate.
>>
>> So If this makes sense may be we could try to define a common/shared
>> database of pre-defined "tiling schemes"/"grid sets" for most EPSG
>> CRS's based on the EPSG database information?
> I like the idea and the EPSG database would be a good starting point to create these grid definitions, but I see one issue with it: How do you define the resolutions of the levels? For some people the resolution doesn't matter and they can live with factor 2 between each level, but most of our customers have GIS clients that show maps at fixed scales (at least often enough to cache at these scales). I think it is nearly impossible to come up with a common set of scales and even if, these scales will result in varying resolutions with different software (72/96/OGC dpi).
I know the dutch have defined a 'standard' tiling scheme: 
http://www.geonovum.nl/sites/default/files/Nederlandse_richtlijn_tiling_-_versie_1.0.pdf 
(dutch) they purposely deviated from the 'standard' scales of the dutch 
topographic survey (1:10k, 1:25k, 1:50k, 1:250k). They recognised that 
tiling is mainly meant for on screen rendering and that 'scale' is not 
as fixed a thing since resolutions of displays vary (as noted by oliver).
Since we now have this guideline, we can recommend our customers to 
either use this scheme or the spherical mercator one, since it will be 
(more) interoperable with all other services within the Netherlands.

We can come up with some sensible scheme's and these scheme's should be 
shared between the various applications, but I think to define an actual 
'national' tiling scheme, like there are national projections 
(Rijksdriehoek EPSG:28992 for .nl), should be done by the national 
authority to be accepted outside our community.

regards,
Steven