I need to know what is the current center of the Cesium Map.
I tried to use viewer.camera.position but it always gives the same z value (x: 16921255.101297915, y: 5578093.302269477, z: 12756274) and I'm not sure about the x and y values. Are they in meters?
Thanks a lot!
EDIT: Solution
With all the help I got (thanks!) I put this together:
getPosition(){
if (viewer.scene.mode == 3) {
var windowPosition = new Cesium.Cartesian2(viewer.container.clientWidth / 2, viewer.container.clientHeight / 2);
var pickRay = viewer.scene.camera.getPickRay(windowPosition);
var pickPosition = viewer.scene.globe.pick(pickRay, viewer.scene);
var pickPositionCartographic = viewer.scene.globe.ellipsoid.cartesianToCartographic(pickPosition);
console.log(pickPositionCartographic.longitude * (180 / Math.PI));
console.log(pickPositionCartographic.latitude * (180 / Math.PI));
} else if (viewer.scene.mode == 2) {
var camPos = viewer.camera.positionCartographic;
console.log(camPos.longitude * (180 / Math.PI));
console.log(camPos.latitude * (180 / Math.PI));
}
};
This function gives longitude/latitude coordinates in degrees.
viewer.camera.position gives you the position at which camera is located in X,Y,Z coordinates in meters in relation to earth center.
Depending on which scene mode you are using approach is different:
SCENE3D:
In order to see at what is the camera looking at you need to get the intersect point of camera's pick ray and map.
function getMapCenter() {
var windowPosition = new Cesium.Cartesian2(viewer.container.clientWidth / 2, viewer.container.clientHeight / 2);
var pickRay = viewer.scene.camera.getPickRay(windowPosition);
var pickPosition = viewer.scene.globe.pick(pickRay, viewer.scene);
var pickPositionCartographic = viewer.scene.globe.ellipsoid.cartesianToCartographic(pickPosition);
console.log(pickPositionCartographic.longitude * (180/Math.PI));
console.log(pickPositionCartographic.latitude * (180/Math.PI));
}
Based on this thread.
Also try to check if camera is looking at the map, and not a the sky.
SCENE2D:
This is a simple 2D view with camera pointing directly down.
From docs:
2D mode. The map is viewed top-down with an orthographic projection
var camPos = viewer.camera.positionCartographic;
console.log(camPos.longitude * (180/Math.PI));
console.log(camPos.latitude * (180/Math.PI));
Remaining case is 2.5D or COLUMBUS_VIEW
I have drawn a rectangle marker in the map using leaflet. When i zoom in the map the rectangle marker automatically becomes large. But i don't want that. I want that the width of the marker may increase but the height of the marker should remain as it is. How can i do that in leaflet? Here is my code
map2.setView([outputf[0].lat,outputf[0].lon], 9);
// Code to rectange
var latLng = L.latLng(outputf[0].lat, outputf[0].lon);
var currentPoint = map2.latLngToContainerPoint(latLng);
var W = 20;
var H = 12*map2.getZoom()/9;
var xDifference = W / 2;
var yDifference = H / 2;
var southWest = L.point((currentPoint.x - xDifference), (currentPoint.y - yDifference));
var northEast = L.point((currentPoint.x + xDifference), (currentPoint.y + yDifference));
var bounds = L.latLngBounds(map2.containerPointToLatLng(southWest),map2.containerPointToLatLng(northEast));
L.rectangle(bounds,{color: "black", opacity:.6, weight: 2,fillColor: "red",id:dir}).bindPopup('WorkZone is: '+output[ax].key).addTo(map2).on('click', onClick);
W denotes the width and H denotes the Height of rectangle. I was trying with
var H = 12*map2.getZoom()/9;
But it doesn't work at all.
I am using Bing maps to display markers on the map.
Now I am adding a functionality where I allow the user to draw a circle around any marker of his choice and let him specify the radius of the circle in kilometres.
I want the circle(polygon) to contain markers within the latitude/longitude bounds of that circle and the markers outside of the circle to disappear.
How do I achieve this?
You have provided no code.
However, I was involved in a similar project so I have an idea what you are talking about.
SInce you are looking for custom polygon you can have a look at this link.
Another way to do it would be to the x,y coordinates of the marker and using Javascript to draw a circle around the map.
THIS JAVASCRIPT SHOULD HELP
function drawCircle(radius, origin) {
var RadPerDeg = Math.PI / 180;
var earthRadius = 3959;
var lat = origin.latitude * RadPerDeg;
var lon = origin.longitude * RadPerDeg;
var locs = new Array();
var AngDist = parseFloat(radius) / earthRadius;
for (x = 0; x <= 360; x++) { //making a 360-sided polygon
var pLatitude, pLongitude;
// With a nice, flat earth we could just say p2.Longitude = lon * sin(brng) and p2.Latitude = lat * cos(brng)
// But it ain't, so we can't. See http://www.movable-type.co.uk/scripts/latlong.html
brng = x * RadPerDeg;
pLatitude = Math.asin(Math.sin(lat) * Math.cos(AngDist) + Math.cos(lat) * Math.sin(AngDist) * Math.cos(brng)); //still in radians
pLongitude = lon + Math.atan2(Math.sin(brng) * Math.sin(AngDist) * Math.cos(lat), Math.cos(AngDist) - Math.sin(lat) * Math.sin(pLatitude));
pLatitude = pLatitude / RadPerDeg;
pLongitude = pLongitude / RadPerDeg;
locs.push(new MM.Location(pLatitude, pLongitude));
};
circle = new MM.Polyline(locs, { visible: true, strokeThickness: 2, strokeDashArray: "1", strokeColor: new MM.Color(200, 0, 0, 200, 0) });
map.entities.push(circle);
};
I'm looking for a way to calculate the zoom level for a given bounds using the Google Maps V3 API, similar to getBoundsZoomLevel() in the V2 API.
Here is what I want to do:
// These are exact bounds previously captured from the map object
var sw = new google.maps.LatLng(42.763479, -84.338918);
var ne = new google.maps.LatLng(42.679488, -84.524313);
var bounds = new google.maps.LatLngBounds(sw, ne);
var zoom = // do some magic to calculate the zoom level
// Set the map to these exact bounds
map.setCenter(bounds.getCenter());
map.setZoom(zoom);
// NOTE: fitBounds() will not work
Unfortunately, I can't use the fitBounds() method for my particular use case. It works well for fitting markers on the map, but it does not work well for setting exact bounds. Here is an example of why I can't use the fitBounds() method.
map.fitBounds(map.getBounds()); // not what you expect
Thanks to Giles Gardam for his answer, but it addresses only longitude and not latitude. A complete solution should calculate the zoom level needed for latitude and the zoom level needed for longitude, and then take the smaller (further out) of the two.
Here is a function that uses both latitude and longitude:
function getBoundsZoomLevel(bounds, mapDim) {
var WORLD_DIM = { height: 256, width: 256 };
var ZOOM_MAX = 21;
function latRad(lat) {
var sin = Math.sin(lat * Math.PI / 180);
var radX2 = Math.log((1 + sin) / (1 - sin)) / 2;
return Math.max(Math.min(radX2, Math.PI), -Math.PI) / 2;
}
function zoom(mapPx, worldPx, fraction) {
return Math.floor(Math.log(mapPx / worldPx / fraction) / Math.LN2);
}
var ne = bounds.getNorthEast();
var sw = bounds.getSouthWest();
var latFraction = (latRad(ne.lat()) - latRad(sw.lat())) / Math.PI;
var lngDiff = ne.lng() - sw.lng();
var lngFraction = ((lngDiff < 0) ? (lngDiff + 360) : lngDiff) / 360;
var latZoom = zoom(mapDim.height, WORLD_DIM.height, latFraction);
var lngZoom = zoom(mapDim.width, WORLD_DIM.width, lngFraction);
return Math.min(latZoom, lngZoom, ZOOM_MAX);
}
Demo on jsfiddle
Parameters:
The "bounds" parameter value should be a google.maps.LatLngBounds object.
The "mapDim" parameter value should be an object with "height" and "width" properties that represent the height and width of the DOM element that displays the map. You may want to decrease these values if you want to ensure padding. That is, you may not want map markers within the bounds to be too close to the edge of the map.
If you are using the jQuery library, the mapDim value can be obtained as follows:
var $mapDiv = $('#mapElementId');
var mapDim = { height: $mapDiv.height(), width: $mapDiv.width() };
If you are using the Prototype library, the mapDim value can be obtained as follows:
var mapDim = $('mapElementId').getDimensions();
Return Value:
The return value is the maximum zoom level that will still display the entire bounds. This value will be between 0 and the maximum zoom level, inclusive.
The maximum zoom level is 21. (I believe it was only 19 for Google Maps API v2.)
Explanation:
Google Maps uses a Mercator projection. In a Mercator projection the lines of longitude are equally spaced, but the lines of latitude are not. The distance between lines of latitude increase as they go from the equator to the poles. In fact the distance tends towards infinity as it reaches the poles. A Google Maps map, however, does not show latitudes above approximately 85 degrees North or below approximately -85 degrees South. (reference) (I calculate the actual cutoff at +/-85.05112877980658 degrees.)
This makes the calculation of the fractions for the bounds more complicated for latitude than for longitude. I used a formula from Wikipedia to calculate the latitude fraction. I am assuming this matches the projection used by Google Maps. After all, the Google Maps documentation page I link to above contains a link to the same Wikipedia page.
Other Notes:
Zoom levels range from 0 to the maximum zoom level. Zoom level 0 is the map fully zoomed out. Higher levels zoom the map in further. (reference)
At zoom level 0 the entire world can be displayed in an area that is 256 x 256 pixels. (reference)
For each higher zoom level the number of pixels needed to display the same area doubles in both width and height. (reference)
Maps wrap in the longitudinal direction, but not in the latitudinal direction.
A similar question has been asked on the Google group: http://groups.google.com/group/google-maps-js-api-v3/browse_thread/thread/e6448fc197c3c892
The zoom levels are discrete, with the scale doubling in each step. So in general you cannot fit the bounds you want exactly (unless you are very lucky with the particular map size).
Another issue is the ratio between side lengths e.g. you cannot fit the bounds exactly to a thin rectangle inside a square map.
There's no easy answer for how to fit exact bounds, because even if you are willing to change the size of the map div, you have to choose which size and corresponding zoom level you change to (roughly speaking, do you make it larger or smaller than it currently is?).
If you really need to calculate the zoom, rather than store it, this should do the trick:
The Mercator projection warps latitude, but any difference in longitude always represents the same fraction of the width of the map (the angle difference in degrees / 360). At zoom zero, the whole world map is 256x256 pixels, and zooming each level doubles both width and height. So after a little algebra we can calculate the zoom as follows, provided we know the map's width in pixels. Note that because longitude wraps around, we have to make sure the angle is positive.
var GLOBE_WIDTH = 256; // a constant in Google's map projection
var west = sw.lng();
var east = ne.lng();
var angle = east - west;
if (angle < 0) {
angle += 360;
}
var zoom = Math.round(Math.log(pixelWidth * 360 / angle / GLOBE_WIDTH) / Math.LN2);
For version 3 of the API, this is simple and working:
var latlngList = [];
latlngList.push(new google.maps.LatLng(lat, lng));
var bounds = new google.maps.LatLngBounds();
latlngList.each(function(n) {
bounds.extend(n);
});
map.setCenter(bounds.getCenter()); //or use custom center
map.fitBounds(bounds);
and some optional tricks:
//remove one zoom level to ensure no marker is on the edge.
map.setZoom(map.getZoom() - 1);
// set a minimum zoom
// if you got only 1 marker or all markers are on the same address map will be zoomed too much.
if(map.getZoom() > 15){
map.setZoom(15);
}
Dart Version:
double latRad(double lat) {
final double sin = math.sin(lat * math.pi / 180);
final double radX2 = math.log((1 + sin) / (1 - sin)) / 2;
return math.max(math.min(radX2, math.pi), -math.pi) / 2;
}
double getMapBoundZoom(LatLngBounds bounds, double mapWidth, double mapHeight) {
final LatLng northEast = bounds.northEast;
final LatLng southWest = bounds.southWest;
final double latFraction = (latRad(northEast.latitude) - latRad(southWest.latitude)) / math.pi;
final double lngDiff = northEast.longitude - southWest.longitude;
final double lngFraction = ((lngDiff < 0) ? (lngDiff + 360) : lngDiff) / 360;
final double latZoom = (math.log(mapHeight / 256 / latFraction) / math.ln2).floorToDouble();
final double lngZoom = (math.log(mapWidth / 256 / lngFraction) / math.ln2).floorToDouble();
return math.min(latZoom, lngZoom);
}
Here a Kotlin version of the function:
fun getBoundsZoomLevel(bounds: LatLngBounds, mapDim: Size): Double {
val WORLD_DIM = Size(256, 256)
val ZOOM_MAX = 21.toDouble();
fun latRad(lat: Double): Double {
val sin = Math.sin(lat * Math.PI / 180);
val radX2 = Math.log((1 + sin) / (1 - sin)) / 2;
return max(min(radX2, Math.PI), -Math.PI) /2
}
fun zoom(mapPx: Int, worldPx: Int, fraction: Double): Double {
return floor(Math.log(mapPx / worldPx / fraction) / Math.log(2.0))
}
val ne = bounds.northeast;
val sw = bounds.southwest;
val latFraction = (latRad(ne.latitude) - latRad(sw.latitude)) / Math.PI;
val lngDiff = ne.longitude - sw.longitude;
val lngFraction = if (lngDiff < 0) { (lngDiff + 360) / 360 } else { (lngDiff / 360) }
val latZoom = zoom(mapDim.height, WORLD_DIM.height, latFraction);
val lngZoom = zoom(mapDim.width, WORLD_DIM.width, lngFraction);
return minOf(latZoom, lngZoom, ZOOM_MAX)
}
None of the highly upvoted answers worked for me. They threw various undefined errors and ended up calculating inf/nan for angles. I suspect perhaps the behavior of LatLngBounds has changed over time. In any case, I found this code to work for my needs, perhaps it can help someone:
function latRad(lat) {
var sin = Math.sin(lat * Math.PI / 180);
var radX2 = Math.log((1 + sin) / (1 - sin)) / 2;
return Math.max(Math.min(radX2, Math.PI), -Math.PI) / 2;
}
function getZoom(lat_a, lng_a, lat_b, lng_b) {
let latDif = Math.abs(latRad(lat_a) - latRad(lat_b))
let lngDif = Math.abs(lng_a - lng_b)
let latFrac = latDif / Math.PI
let lngFrac = lngDif / 360
let lngZoom = Math.log(1/latFrac) / Math.log(2)
let latZoom = Math.log(1/lngFrac) / Math.log(2)
return Math.min(lngZoom, latZoom)
}
Thanks, that helped me a lot in finding the most suitable zoom factor to correctly display a polyline.
I find the maximum and minimum coordinates among the points I have to track and, in case the path is very "vertical", I just added few lines of code:
var GLOBE_WIDTH = 256; // a constant in Google's map projection
var west = <?php echo $minLng; ?>;
var east = <?php echo $maxLng; ?>;
*var north = <?php echo $maxLat; ?>;*
*var south = <?php echo $minLat; ?>;*
var angle = east - west;
if (angle < 0) {
angle += 360;
}
*var angle2 = north - south;*
*if (angle2 > angle) angle = angle2;*
var zoomfactor = Math.round(Math.log(960 * 360 / angle / GLOBE_WIDTH) / Math.LN2);
Actually, the ideal zoom factor is zoomfactor-1.
Since all of the other answers seem to have issues for me with one or another set of circumstances (map width/height, bounds width/height, etc.) I figured I'd put my answer here...
There was a very useful javascript file here: http://www.polyarc.us/adjust.js
I used that as a base for this:
var com = com || {};
com.local = com.local || {};
com.local.gmaps3 = com.local.gmaps3 || {};
com.local.gmaps3.CoordinateUtils = new function() {
var OFFSET = 268435456;
var RADIUS = OFFSET / Math.PI;
/**
* Gets the minimum zoom level that entirely contains the Lat/Lon bounding rectangle given.
*
* #param {google.maps.LatLngBounds} boundary the Lat/Lon bounding rectangle to be contained
* #param {number} mapWidth the width of the map in pixels
* #param {number} mapHeight the height of the map in pixels
* #return {number} the minimum zoom level that entirely contains the given Lat/Lon rectangle boundary
*/
this.getMinimumZoomLevelContainingBounds = function ( boundary, mapWidth, mapHeight ) {
var zoomIndependentSouthWestPoint = latLonToZoomLevelIndependentPoint( boundary.getSouthWest() );
var zoomIndependentNorthEastPoint = latLonToZoomLevelIndependentPoint( boundary.getNorthEast() );
var zoomIndependentNorthWestPoint = { x: zoomIndependentSouthWestPoint.x, y: zoomIndependentNorthEastPoint.y };
var zoomIndependentSouthEastPoint = { x: zoomIndependentNorthEastPoint.x, y: zoomIndependentSouthWestPoint.y };
var zoomLevelDependentSouthEast, zoomLevelDependentNorthWest, zoomLevelWidth, zoomLevelHeight;
for( var zoom = 21; zoom >= 0; --zoom ) {
zoomLevelDependentSouthEast = zoomLevelIndependentPointToMapCanvasPoint( zoomIndependentSouthEastPoint, zoom );
zoomLevelDependentNorthWest = zoomLevelIndependentPointToMapCanvasPoint( zoomIndependentNorthWestPoint, zoom );
zoomLevelWidth = zoomLevelDependentSouthEast.x - zoomLevelDependentNorthWest.x;
zoomLevelHeight = zoomLevelDependentSouthEast.y - zoomLevelDependentNorthWest.y;
if( zoomLevelWidth <= mapWidth && zoomLevelHeight <= mapHeight )
return zoom;
}
return 0;
};
function latLonToZoomLevelIndependentPoint ( latLon ) {
return { x: lonToX( latLon.lng() ), y: latToY( latLon.lat() ) };
}
function zoomLevelIndependentPointToMapCanvasPoint ( point, zoomLevel ) {
return {
x: zoomLevelIndependentCoordinateToMapCanvasCoordinate( point.x, zoomLevel ),
y: zoomLevelIndependentCoordinateToMapCanvasCoordinate( point.y, zoomLevel )
};
}
function zoomLevelIndependentCoordinateToMapCanvasCoordinate ( coordinate, zoomLevel ) {
return coordinate >> ( 21 - zoomLevel );
}
function latToY ( lat ) {
return OFFSET - RADIUS * Math.log( ( 1 + Math.sin( lat * Math.PI / 180 ) ) / ( 1 - Math.sin( lat * Math.PI / 180 ) ) ) / 2;
}
function lonToX ( lon ) {
return OFFSET + RADIUS * lon * Math.PI / 180;
}
};
You can certainly clean this up or minify it if needed, but I kept the variable names long in an attempt to make it easier to understand.
If you are wondering where OFFSET came from, apparently 268435456 is half of earth's circumference in pixels at zoom level 21 (according to http://www.appelsiini.net/2008/11/introduction-to-marker-clustering-with-google-maps).
Valerio is almost right with his solution, but there is some logical mistake.
you must firstly check wether angle2 is bigger than angle, before adding 360 at a negative.
otherwise you always have a bigger value than angle
So the correct solution is:
var west = calculateMin(data.longitudes);
var east = calculateMax(data.longitudes);
var angle = east - west;
var north = calculateMax(data.latitudes);
var south = calculateMin(data.latitudes);
var angle2 = north - south;
var zoomfactor;
var delta = 0;
var horizontal = false;
if(angle2 > angle) {
angle = angle2;
delta = 3;
}
if (angle < 0) {
angle += 360;
}
zoomfactor = Math.floor(Math.log(960 * 360 / angle / GLOBE_WIDTH) / Math.LN2) - 2 - delta;
Delta is there, because i have a bigger width than height.
map.getBounds() is not momentary operation, so I use in similar case event handler. Here is my example in Coffeescript
#map.fitBounds(#bounds)
google.maps.event.addListenerOnce #map, 'bounds_changed', =>
#map.setZoom(12) if #map.getZoom() > 12
Work example to find average default center with react-google-maps on ES6:
const bounds = new google.maps.LatLngBounds();
paths.map((latLng) => bounds.extend(new google.maps.LatLng(latLng)));
const defaultCenter = bounds.getCenter();
<GoogleMap
defaultZoom={paths.length ? 12 : 4}
defaultCenter={defaultCenter}
>
<Marker position={{ lat, lng }} />
</GoogleMap>
The calculation of the zoom level for the longitudes of Giles Gardam works fine for me.
If you want to calculate the zoom factor for latitude, this is an easy solution that works fine:
double minLat = ...;
double maxLat = ...;
double midAngle = (maxLat+minLat)/2;
//alpha is the non-negative angle distance of alpha and beta to midangle
double alpha = maxLat-midAngle;
//Projection screen is orthogonal to vector with angle midAngle
//portion of horizontal scale:
double yPortion = Math.sin(alpha*Math.pi/180) / 2;
double latZoom = Math.log(mapSize.height / GLOBE_WIDTH / yPortion) / Math.ln2;
//return min (max zoom) of both zoom levels
double zoom = Math.min(lngZoom, latZoom);
For swift version
func getBoundsZoomLevel(bounds: GMSCoordinateBounds, mapDim: CGSize) -> Double {
var bounds = bounds
let WORLD_DIM = CGSize(width: 256, height: 256)
let ZOOM_MAX: Double = 21.0
func latRad(_ lat: Double) -> Double {
let sin2 = sin(lat * .pi / 180)
let radX2 = log10((1 + sin2) / (1 - sin2)) / 2
return max(min(radX2, .pi), -.pi) / 2
}
func zoom(_ mapPx: CGFloat,_ worldPx: CGFloat,_ fraction: Double) -> Double {
return floor(log10(Double(mapPx) / Double(worldPx) / fraction / log10(2.0)))
}
let ne = bounds.northEast
let sw = bounds.southWest
let latFraction = (latRad(ne.latitude) - latRad(sw.latitude)) / .pi
let lngDiff = ne.longitude - sw.longitude
let lngFraction = lngDiff < 0 ? (lngDiff + 360) : (lngDiff / 360)
let latZoom = zoom(mapDim.height, WORLD_DIM.height, latFraction);
let lngZoom = zoom(mapDim.width, WORLD_DIM.width, lngFraction);
return min(latZoom, lngZoom, ZOOM_MAX)
}
Calculate zoom level to display a map including the two cross corners of the area and display the map on a the part of the screen with a specific height.
Two coordinates
max lat/long
min lat/long
Display area in pixels
height
double getZoomLevelNew(context,
double maxLat, double maxLong,
double minLat, double minLong,
double height){
try {
double _zoom;
MediaQueryData queryData2;
queryData2 = MediaQuery.of(context);
double _zLat =
Math.log(
(globals.factor(height) / queryData2.devicePixelRatio / 256.0) *
180 / (maxLat - minLat).abs()) / Math.log(2);
double _zLong =
Math.log((globals.factor(MediaQuery
.of(context)
.size
.width) / queryData2.devicePixelRatio / 256.0) * 360 /
(maxLong - minLong).abs()) / Math.log(2);
_zoom = Math.min(_zLat, _zLong)*globals.zoomFactorNew;
if (_zoom < 0) {
_zoom = 0;
}
return _zoom;
} catch(e){
print("getZoomLevelNew - excep - " + e.toString());
}
In open layers library, below is the method used to convert a screen co ordinate to latitude and longitude. I am not able to figure out the logic this method encapsulates?
getLatLonFromPoint: function (point) {
var center = this.getCenter();
//map center lat/lon
var res = this.getResolution();
//pre defined by the user. Represents the change in lat long per screen unit at the given zoom level
var size = this.getSize();
//this is the width and height of the div in which the map has to be displayed
var delta_x = point.x - (size.w / 2);
var delta_y = point.y - (size.h / 2);
return new OpenLayers.LatLon(
center.lat - delta_y * res,
center.lon + delta_x * res );
}
Can someone please provide some pointers?
The function calculates a latitude and longitude for the specified point based on the current resolution of the map, and the latitude and longitude of the centre point of the current map, and the distance that the chosen point is from the centre of the map.
var center = this.getCenter();
//map center lat/lon
var res = this.getResolution();
//pre defined by the user. Represents the change in lat long ...
var size = this.getSize();
The above bit of code gathers the information needed for the calculation: The centre point of the current map view (which will give us the lat/lon on the centre point), the current map resolution, and the current map size on the users screen (which may be effected by screen size, etc).
Then the calculation goes like this:
//this is the width and height of the div in which the map has to be displayed
var delta_x = point.x - (size.w / 2);
var delta_y = point.y - (size.h / 2);
First take the x co-ord (in pixels) and subtract the width of the map (in pixels). This gives us a new x co-ord in which 0 is the centre pixel of the map. delta-x should now be a pixel value ranging from -(size.w/2) to +(size.w/2).
Then we do the same for y co-ord.
So delta-x and delta-y are now cartesian co-ordinates with the origin at the centre of the map.
return new OpenLayers.LatLon(
center.lat - delta_y * res,
center.lon + delta_x * res );
We need to convert delta-x and delta-y from pixels to lat/lon. First we multiply delta-x and delta-y by the current resolution. This gives us the correct scale but not the correct origin. Adding centre.lat and centre.lon adusts to give us the lat/lon based on the currently displayed map.
Finally the 'new OpenLayers.LatLon' call just wraps the above calculations in a LatLon object, so that it can be returned from the function as a LatLon object.
edit: when working with pixels, an increase in x co-ord normally means 'move right', and an increase in y co-ord normally means 'move up'. On a map, when you increase Longitude, you usually 'move right'. However Latitude is upside down; when you increase Latitude, you normally 'move down' on a map.
Hence Latitude works in the opposite direction to the normal y co-ord scheme on a screen. Hence in the final calculation a minus is used for centre.lat but a plus for centre.lon.
I rearranged the existing comments, added some more and added some white space. Hopefully, you'll find this clearer.
getLatLonFromPoint: function (point) {
// point is the x and y screen coordinate
// map center lat/lon
var center = this.getCenter();
// pre defined by the user. Represents the change in lat long per screen unit at the given zoom level
var res = this.getResolution();
// this is the width and height of the screen (div) in which the map has to be displayed
var size = this.getSize();
// this is the distance of the point from the center of the screen (div)
var delta_x = point.x - (size.w / 2);
var delta_y = point.y - (size.h / 2);
// return the latitude and longitude
// these are calculated from the center lat/lon using the
// screen distances which are scaled (multiplied) by the resolution
return new OpenLayers.LatLon(
center.lat - delta_y * res,
center.lon + delta_x * res );
}
Try this:
map.events.register("mousemove", map, function(e) {
var position = this.events.getMousePosition(e);
var p = map.getLonLatFromPixel(new OpenLayers.Pixel(position.x, position.y));
// your longitude value = p.lon;
// your latitude value = p.lat;
});