[mapserver-users] Pre-defined Zoom In/Zoom Out buttons...

[Joe Bussell]

The beauty of Javascript is that it is by definition open for copiing.
I encourage you to steal whatever you map find useful, though I will not
defend the style.

My idea was to maintain a center position and a level.  The extent is
then calculated as a function of the two.  In my world I have a 6 degree
spread in lat and lon.  My function for the extent is:

 deltaDegree = (max delta) / 2^level

 so at level0 I get 6/2^0 = 6 degrees
    at level1 I get 6/2^1 = 3 degrees
    at level2 I get 6/2^2 = 1.5
    ...
    at level10 I get 6/2^10 = 0.005859375

    I then add/subtract to get my top, bottom, left, and right values.

    In truth I am serving raster images with defined scales, so I need
to match up the result with the raster image.  I have created my rasters
with a given size.  I need to scale my result to match in case I am
presenting an odd shaped image.

var span = level0Span/Math.pow(2,zoomLevel) * mapNameExtentHash[ map ].imageSize / rasterImageSize;
top = lat + span;
bottom = lat - span;
left = lon - span;
right = lon + span;

I have no feeling as to whether this is a good methodology, but it works
for me.  I am presently carriing around values for the present extent,
and am calculating the center point as needed.  This seems like the
wrong choice, so I am migrating to a center-point & level based state.

Cordially,

Joe Bussell
On Time Systems

On Fri, Sep 27, 2002 at 01:46:46PM -0500, Mike Smith wrote:
> I could easily implement a new variable like your "zoomLevel"...I was 
> already thinking that I needed to do that.  And I've got a series of 
> little buttons next to my map that make up a zoom bar like yours, and 
> each button (when clicked) makes a call to a javascript function that 
> I've written called "zoomTo".  So I've got the framework there, I just 
> can't figure out how to set my imgext based on this zoomLevel factor 
> (relative to my map's maximum extent, not the current zoom level).  I'd 
> like to do just what you're doing on your page and have each zoom button 
> present the same imgext and zoom level each time...right now I'm just 
> incrementing the Mapserver variable "zoom" and "zoomsize" and the 
> buttons build on the last zoom, instead of always zooming to a set 
> factor relative to max extent.  I don't want to ask you for your 
> proprietary source code, but can you shed any light on what kind of 
> algorithm I might use to determine the new imgext as a function of max 
> extent and zoomLevel?
> 
> Also, as Puneet pointed out, I will not be overriding the mouse click 
> coordinates with this zoom level, so that the zooming will happen 
> relative to whatever image center the user has already clicked on, and 
> if they haven't clicked anywhere then I'll zoom straight into the 
> centerpoint of my main map.
> 
> Thanks!
> 
> 
> 
> Joe Bussell wrote:
> 
> >www.TrafficDodger.com has just such a zoom bar.  I view it like fliing
> >at different altitudes.  The higher you go, the more area you see and
> >you see less detail.  I use a zoomLevel variable in my javascript.  Each
> >button triggers a call to zoomToLevel with its own arguments.  Regular
> >zoomIn and zoomOut increment and decrement this value.  I then determine
> >the extent based on some function of the zoomLevel and my full scale
> >range.
> >
> >Cordially,
> >
> >Joe Bussell
> >On Time Systems
> >www.TrafficDodger.com
> >
> >
> >
> >On Fri, Sep 27, 2002 at 09:45:06AM -0500, Mike Smith wrote:
> >
> >>Sorry to bother everyone with this, I know this has been addressed 
> >>before in the archives, but I can't seem to find it when I search them.
> >>
> >>I'm trying to enable a Zoom In/Zoom Out bar along side my map (kinda 
> >>like Mapquest or Yahoo maps) where there are buttons that either zoom 
> >>the user in or out to pre-determined levels.  I've got Javascript code 
> >>that zooms in and out ok, but the logic is what's throwing me off.  I 
> >>want the bottom button to always be the same view, not zoom them out 15 
> >>times from whatever their present view is.... it should zoom them out 15 
> >>times from full extent.  I want "full extent" to be the base and each 
> >>button whether it's a zoom in or out button needs to always zoom them to 
> >>the same factor relative to full extent, not their current view...make 
> >>sense?  I know I've seen a Mapserver page before that had this little 
> >>zoom bar that I'm describing, but I can't remember where I saw it. 
> >>Anybody have a web page that does this?  Or example code that handles 
> >>the logic of pre-defined absolute zoom levels (as opposed to the 
> >>relative ones that the CGI variable "zoom" sets)?
> >>
> >>Thanks
> >>
> >>
> >
> 

-- 

Cordially,

Joe Bussell
On Time Systems
www.TrafficDodger.com