Zoom level calculation without using the API

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Jaidev S

unread,
Feb 16, 2010, 12:58:49 AM2/16/10
to Google Maps JavaScript API v3
Hi,

I am working on an enhancement to the map accessible at http://travellr.com/map

The idea is to be able to load the map on a country, and have the map
zoomed to appropriately fit the country. For all countries I have co-
ordinates and bounding boxes already.

The problem is that when using Map.fitBounds(), the zoom level
sometimes isn't quite right - for loading on countries I would like to
limit zoom within a min / max, and places like Russia and Canada
break out of that. After using Map.fitBounds(), I could then check the
zoom level once the zoom_changed event fires, and if required update
it, but that seems quite a clunky multi step process.

Ideally I would like to be able to calculate the zoom level prior to
instantiating the map. However the API v3 does not seem to provide a
way to do this.

I have read some solutions akin to:

// mnode = the map node
// bounds = a bounding box
function best_zoom(bounds, mnode) {
var width = mnode.offsetWidth;
var height = mnode.offsetHeight;

var dlat = Math.abs(bounds.maxY - bounds.minY);
var dlon = Math.abs(bounds.maxX - bounds.minX);

// Center latitude in radians
var clat = Math.PI*(bounds.minY + bounds.maxY)/360.;

var C = 0.0000107288;
var z0 = Math.ceil(Math.log(dlat/(C*height))/Math.LN2);
var z1 = Math.ceil(Math.log(dlon/(C*width*Math.cos(clat)))/
Math.LN2);

return (z1 > z0) ? z1 : z0;
}

However this does not seem to provide the correct zoom levels (they
increase as dlat or dlon increase, shouldn't they go down?) - Please
tell me if I am missing something here? Also, where does the constant
C come from?

Thanks for your help.

- Jai

Philip

unread,
Feb 17, 2010, 8:55:33 PM2/17/10
to Google Maps JavaScript API v3
This is actually a nasty problem when the objects that you are trying
to fit are large. For V2 I used:

function zoomRecenter(map, bounds) {
map.setZoom(map.getBoundsZoomLevel(bounds));
map.panTo(bounds.getCenterMap(map));
}

GLatLngBounds.prototype.getCenterMap = function(m) {
var p = m.getCurrentMapType().getProjection();

var b = new GBounds;
b.extend(p.fromLatLngToPixel(this.getNorthEast(), m.getZoom()));
b.extend(p.fromLatLngToPixel(this.getSouthWest(), m.getZoom()));

return new GLatLng(p.fromPixelToLatLng(b.mid(),
m.getZoom()).lat(), this.getCenter().lng());
}


Note the complexity of finding the *pixel* middle of a GBounds.
However, since it isn't possible (currently) in V3 to get the
projection in use by the current basemap, you are somewhat hosed.

You can star the bug at
http://code.google.com/p/gmaps-api-issues/issues/detail?id=2181&q=apitype%3AJavascript3%20type%3ADefect&sort=-id%20-stars&colspec=ID%20Type%20Status%20Introduced%20Fixed%20Summary%20Internal%20Stars
if you want.

Philip

On Feb 16, 12:58 am, Jaidev S <j...@travellr.com> wrote:
> Hi,
>

> I am working on an enhancement to the map accessible athttp://travellr.com/map

Jaidev S

unread,
Feb 23, 2010, 9:12:18 PM2/23/10
to Google Maps JavaScript API v3
Okay, have worked this out.

C is a value based on your latitude, values I calculated were:

constant_at_0_degrees = 1.406245461070741
constant_at_20_degrees = 1.321415085624082
constant_at_40_degrees = 1.077179995861952
constant_at_60_degrees = 0.703119412486786
constant_at_80_degrees = 0.488332580888611

I found for most countries just using the 60 degree one worked well.

For working out the zoom level to display a bounding box (this is in
ruby but easily translated into JS):

width = width-of-map-element
height = height-of-map-element
bounds_t = top-of-bounding-box
bounds_b = bottom-of-bounding-box
bounds_l = left-of-bounding-box
boubds_r = right-of-bounding-box


dlat = bounds_t - bounds_b

if bounds_l < bounds_r
dlon = bounds_r - bounds_l
else
dlon = 360 - bounds_l + bounds_r
end

z0 = (Math.log(constant_at_60_degrees * height / dlat) /
Math.log(2)).ceil
z1 = (Math.log(constant_at_60_degrees * width / dlon) /
Math.log(2)).ceil

zoom = z1 ? ((z1 > z0) ? z0 : z1) : z0

Happy mapping :)

On Feb 16, 4:58 pm, Jaidev S <j...@travellr.com> wrote:
> Hi,
>

> I am working on an enhancement to the map accessible athttp://travellr.com/map


>
> The idea is to be able to load the map on a country, and have the map
> zoomed to appropriately fit the country. For all countries I have co-
> ordinates and bounding boxes already.
>
> The problem is that whenusingMap.fitBounds(), thezoomlevel
> sometimes isn't quite right - for loading on countries I would like to

> limit  zoomwithin a min / max, and places like Russia and Canada
> break out of that. AfterusingMap.fitBounds(), I could then check thezoomlevelonce the zoom_changed event fires, and if required update


> it, but that seems quite a clunky multi step process.
>

> Ideally I would like to be able to calculate thezoomlevelprior to
> instantiating the map. However theAPIv3 does not seem to provide a


> way to do this.
>
> I have read some solutions akin to:
>
> // mnode = the map node
> // bounds = a bounding box
> function best_zoom(bounds, mnode) {
>    var width = mnode.offsetWidth;
>    var height = mnode.offsetHeight;
>
>    var dlat = Math.abs(bounds.maxY - bounds.minY);
>    var dlon = Math.abs(bounds.maxX - bounds.minX);
>
>    // Center latitude in radians
>    var clat = Math.PI*(bounds.minY + bounds.maxY)/360.;
>
>    var C = 0.0000107288;
>    var z0 = Math.ceil(Math.log(dlat/(C*height))/Math.LN2);
>    var z1 = Math.ceil(Math.log(dlon/(C*width*Math.cos(clat)))/
> Math.LN2);
>
>    return (z1 > z0) ? z1 : z0;
>
> }
>

> However this does not seem to provide the correctzoomlevels (they

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