[OpenLayers-Users] Saving with a Google Maps base layer

[Tom B]

OK, this has been pretty straightforward (thanks for the tips, Gregor).  The
one issue I'm having is calculating the latitude value of the tiles' centers
while accounting for the change in distance per latitude degree while
traveling north-south.  The code I'm using now calculates a "latitude
degrees per pixel" value, which is an alright approximation for the whole
map at very high zoom levels, but at lower zoom levels it isn't accurate,
and the resulting tile rows begin to drift apart.  I have very little GIS
experience, so I'm a bit lost on where to start with calculating these
centers accurately.  

Any ideas of how to approach this?  I'm guessing that trig-related
expressions will be involved.

To show how this is currently set up, my code is:

// $width and $height - the pixel dimensions of the map
// $bounds - the EPSG:4326 BBOX of the map
function add_gmaps_layer($image, $width, $height, $bounds, $zoom, $key,
$temp_dir){
	$base_url =
'http://maps.google.com/staticmap?key='.$key.'&sensor=false&size=640x640&zoom='.$zoom;
	$num_cols = ceil($width/640);
	$num_rows = ceil($height/640);
	list($left, $bottom, $right, $top) = split(',', $bounds);
	$lon_per_px = abs($right-$left)/$width;
	$lat_per_px = abs($top-$bottom)/$height;
	for($i=0; $i<$num_cols; $i++){
		for($j=0; $j<$num_rows; $j++){
			// Calculate the tile's center, relative to the bottom right corner of
the map
			$center_x = 320 + ($i*640);
			$center_y = 320 + ($j*640);
			// Calculate the lon, lat of the tile
			$center_lon = $right - ($center_x*$lon_per_px);
			$center_lat = $bottom + ($center_y*$lat_per_px);
			$url = $base_url.'&center='.$center_lat.','.$center_lon;
			$x = $width - (($i+1)*640);
			$y = $height - (($j+1)*640);
			// Blit the tile onto the main image width a displacement of $x, $y 
			$image = blit_from_url($image, $url, $x, $y, $temp_dir);
		}
	}
	return $image;
}


Gregor at HostGIS wrote:
> 
> Tom B wrote:
>> Has anyone had a chance to try this out?  Gregor, any luck with getting
>> R&D
>> funding, by any chance?  If not, I might go ahead and take a crack at it,
>> since it is something our map will eventually need.
> 
> The customer wasn't up for it when I presented it to him, so I have 
> nothing to show on this. Sorry.
> 
> The way I figure to render it, would be:
> 
> Take image width, divide by 640 rounding up.
> Take image height, divide by 640 rounding up.
> foreach w in width:
>    foreach h in height:
>      figure up center latlng (320,320) based on bbox and pixel position
>      figure URL
>      fetch image
>      blit it onto the canvas
> 
> I have existing code in the Wiki ("Printing" page) for doing this in 
> PHP, using URLs that are provided by the client. That at least shows the 
> blitting and alpha code.
> 
> -- 
> HostGIS, Open Source solutions for the global GIS community
> Greg Allensworth - SysAdmin, Programmer, GIS Person, Security
> Network+   Server+   A+   Security+
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> 
> 

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