AW: [OpenLayers-Users] Creating an application with pre-defined mapscales/resolutions

[Robert Buckley]

Hi,

Thanks.

"The differences between the different projections is, that the meter doesn't describe the real length." ...ok I think I see...you mean that google distorted the projection so it fits nicely in a square whereas the others are "real" projected coordinate systems?


But what I was really asking is that why have I seen so many different resolutions for various maps.

cheers,

Rob

.


________________________________
 Von: Arnd Wippermann <arnd.wippermann at web.de>
An: 'Robert Buckley' <robertdbuckley at yahoo.com> 
Cc: users at openlayers.org 
Gesendet: 19:01 Dienstag, 6.Dezember 2011
Betreff: AW: [OpenLayers-Users] Creating an application with pre-defined mapscales/resolutions
 

Hi,
 
it should be correct. 1 meter is always 1 meter. The 
differences between the different projections is, that the meter doesn't 
describe the real length.
 
EPSG:25832 is a projection, where a meter on map is a meter in 
reality.
For i.e. EPSG:900913 (Google Projection) it's 
not.
 
Distance Düsseldorf - Hannover 
 
EPSG:25832 : 240.9911275436498
EPSG:900913 : 
389.3661105393922

 
Arnd


________________________________
 Von: openlayers-users-bounces at lists.osgeo.org 
[mailto:openlayers-users-bounces at lists.osgeo.org] Im Auftrag von Robert 
Buckley
Gesendet: Dienstag, 6. Dezember 2011 08:48
An: users at openlayers.org
Betreff: [OpenLayers-Users] Creating an 
application with pre-defined mapscales/resolutions


Hi,

I would 
like to have a simple application which delivers wms layers in 
scales/resolutions which are not the usual OSM/Google messy scales, but rather 
include 1:1000 000 / 1:500 000 / 1:250 000 / 1:100 000 / 1: 50 0000/ 1:10 000, 
1: 5000 and 1:2,500. To make this even more difficult I need to do this in 
EPSG:25832 the european standard ETRS/ UTM zone 32 projected CRS with units of 
meters

As far 
as I have understood, this is fully dependant on the basemap resolutions. As 
this is pure mathematical I would like to show what I think I have learnt and 
wait for comments....


R 
= resolution 
S 
= scale as denominator
IPU = OpenLayers.INCHES_PER_UNIT[units] = 39.3701 inches in a 
meter
DPI = OpenLayers.DOTS_PER_INCH = 25.4 / 0.28 = 
90.71428571428571 ( 1 inch = 25.4 mm / standardized pixel size of 
0.28x0.28mm)
this must be set in the application code "OpenLayers.DOTS_PER_INCH = 25.4 / 0.28" = 90.7142   otherwise, the scale factor would be 72 / 90.7143 = 0.7937.

R= 
S / IPU * DPI

using this formular, we are basically 
working out how many pixels there are in 1 meter of screen, to so find out how 
many pixels there are in 1000000 meters of screen we use...

[Scale 1:1000 000]

R = 1000000  / (39.2701 * 90.71428571428571) 
R =1000000  / 3562.359071428571260271
R = 280.712859077111976006937033


[Scale 1:500000]

R = 500000  / (39.2701 * 90.71428571428571) 
R =500000  / 3562.359071428571260271
R = 140.356429538555988003468517

...etc for the 1:250 000 / 1:100 000 ...



What bothers me though, is that every EPSG code with units of meters will have the same resolutions. Is this correct?


sources: 
http://geowebcache.org/docs/current/concepts/gridsets.html
http://wiki.osgeo.org/wiki/WMS_Tiling_Client_Recommendation
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