[openstreetmap-website] Replace scale with zoom level in Export tab (#368)

Ilya Zverev notifications at github.com
Sun Jul 21 07:27:15 UTC 2013


First: current scale field is counterintuitive, because it doesn't allow a user to predict what they would be getting. There is no option of "give me the exact same picture I see". Zoom coefficient (that you propose) or zoom level (that I propose) would fix that.

A scale, as in paper maps, is irrelevant in web mapping. Everyone has different monitors, and when it comes to printing, you can't really guess. So the scale number in the Export tab is just a number for a mapnik renderer, which affects a) size of an image a user would receive; b) map detalization. I'm not sure why those two are connected: image size has no meaning for vector exports (pdf and svg).

I have just made an image for myself using the Export tab. I've put some arbitrary number, and for map view of z15 got a small map image of zoom level around 13.5, judging by features present. There are almost no street names, and a railway station isn't labelled. If I wanted to have the same image I was seeing, but 4 times smaller, I'd be disappointed: it is not the same. So, not only image size was changed (which zoom coefficient you are proposing implies), but also a number of features present on a map extract.

Now, you assume zoom level is an unfamiliar concept for regular users. I agree: it is, as well as scale, and even zoom coefficient (why not enter target image size, and instead make a silly calculator: there is a size displayed near the field, so it's "enter 2 and see those numbers multiplied by 2"?). But in terms of control and predictability I am sure zoom level is the best we can do. Target image size is displayed: a user can tweak zoom level to get dimensions they need. We can emphasize the floating point by putting a default value with it: "15.0". Users are familiar with discrete zoom levels, since every popular web map has them. User do know that map presentation differs for different zoom levels, and they either would be happy with the picture they see (of a current zoom level), or would wish to specify the required detallization: e.g. when selecting an area that is larger than their screen size, or for printing with a high (or low) dpi.

The other way we could give a user such control, is to have two input fields: for zoom level (integer) and for dpi (disabled for vector output). But I doubt that would pass, because it complicates the form and is less understandable if the image is needed not for printing. 

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/openstreetmap-website/issues/368#issuecomment-21306304
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