[OpenLayers-Users] Need help understanding UTM

[Barend Köbben]

Hi Linda,
 
> I guess I am just an idiot but I just don't get it.  I really appreciate you taking 
> the time to try and explain but I just don't get it.
 
I teach MSc students in GeoInformatics that still don't get it, so don't feel embarrassed. It's just that 'scale' and 'projection' are sometimes hard to get to grips with.
 
> Yes I want a world map.
> Then when they get down to a certain resolution I want to display UTM.  
> What would be really cool is if I could control the box at that level too.  
> It needs to be 1200 UTM meters square, 2400 UTM Meters square and 
> 4800 Meters square for the zooms. 
 
This is part of your problem. You cannot expect to use data that is useful for a very large scale (4800 msq in UTM on a screen boils down to something like a 1:25000 scale) to also be useable for a small scale world map (on screen something like a 1: 200000000 million scale).  That's just plain impossible. Mapping sites like Google Maps use diffferent datasets for different scale ranges, you will have to do that too... 
 
> But if I use a world map it is in either spherical mercator or lat/lon.  So 
> right now I am hiding and showing a layer.  Taking the lat/lon converting 
> to UTM when they get to that level and returning that image.  
 
Google Maps and co. do exactly that: Use latlon until a certains cale and then switch to projected meter maps.  
 
> slow and it just isn't working right.  It would be easier if the entire map was 
> in something like meters that I could go directly to UTM without the lat/lon 
> stuff in the middle.  
 
The slowness is likely  because of you using the same data set to have both types of maps and doping re-projections all the time. You'll have to use pre-made datasets for the different scale levels for decent performance. 
 
> The UTM image is just not the same image as the lat/lon 
> image.  Even though the coordinates are directly converted.  I am so frustrated.  
 
Of course they are not the same. Projection of a spherical object (Earth) to a flat cartesian object (projected map) always will introduce distortions. Imagine peeling an orange and flattening the skin. It will distort (and probably break). 
 
> I need a road map. 
 
So make yourself one :-) But don't try to do it from a world dataset. Or maybe better: use an existing road map, where people have done the hard stuff for you already. Can you use Google Maps as a background layer in your OpenLayers stack? If you need the actual road DATA, you could consider the free OpenStreetMap (openstreetmap.org <http://www.osm.org/> ), although their coverage is not so good for some parts of the world. Otherwise you might have to buy some data from teh likes of NavTech or TeleAtlas...
 
 
-- 
Barend Köbben 
International Institute for Geo-information Sciences and  Earth Observation (ITC) 
PO Box 6, 7500AA Enschede (The Netherlands) 
ph: +31-(0)534874253; mobile: +31-(0)622344955