Being new to GIS and Open layers I spent a long time trying to make sense of what resolution is. Finally, I think I have a fair idea of what resolution means in open layers world.

Resolution is nothing but a way of saying how much your map units vary as you move on the computer screen.

Say, you want to represent a map bounded by ( -180,-90,180,90) in a window of dimension 256×256. Now, how do you calculate the resolution?

Since the map extends from -180 to +180, you have to fit a total of 360 degrees in a screen of size 256. That is, your resolution is 360/256=1.40625.

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Thanks for your explication, but that is in the projection EPSG:4326, but in the projection EPSG:900913:

Resolution = 40,075,016.686 (equatorical circumference in miles) / 256 = 156543,0339

😉

Sorry! The Earth has 40,075,016.686 equatorical circunference in meters so:

Resolution = 40,075,016.686 (equatorical circumference in meters) / 256 = 156543,0339

Thanks.

Excuse me. I have seen this example here http://gis.ibbeck.de/ginfo/apps/OLExamples/OL27/examples/ExternalGraphicOverlay/ExternalGraphicOverlay.asp and I do not understand the values of resolutions set by each image. For example, if I have an image 400×400 pixels which represents an area of 400 x 400 kilometers, how can I calculate the correct width when I zoom in/out?

imagewidth = 400 * xxx / map.getResolution()

how can I calculate xxx?