Tuesday, August 30, 2005

WMS overlays and GoogleMap Transparency

Following are some notes on my experiences in working with WMS layers and GoogleMaps API. Am posting these notes here as I spent alot of time testing in the last couple of weeks on this and maybe this will help someone else out. Following is an image which illustrates what i'm talking about:

These ideas are not definitive, just some general notes about how to get around.

1) WMS layering on Googlemaps. Some posts reccomended having two map objects overlaying in separate divs. I drove myself nuts getting the maps to sync and things become more problematic when implementing multiple events to trigger different actions. Plus, this was all super slow. Hence, I opted for combining WMS layers into a single map object.

2) I integrated a combination of Brian Flood's WMS code and Kyle Mulka's suggestions for handling overlays into a single function to handle WMS overlays (can overlay WMS ontop of GoogleMaps data or other WMS layers such as terraserver). I reccomend working with scripts they have posted rather than anything i have done.

3) When Google handles overlays in the map object the Style sheets force transparent images to become opaque. Hence filled polygons with translucent properties block out everything. Worse, Google doesn't assign unique id's to any of the divs they create to simplify working with divs. To get around this, I pointed my javascript DOM to the div container for my map container and then searched all child nodes for references to my overlay WMS service and then dynamically changed the style properties. Here is the JS function which updates the appropriate JS elements:
// adjust Opacity
function adjustOpacity(p_pctOpacity) {
if (p_pctOpacity < .01) p_pctOpacity=.01 if (p_pctOpacity >1) p_pctOpacity=1;
// adjust opacity of overlay layers
var JStopmap=document.getElementById('topmap').firstChild.childNodes;
// loop all elements (google tiles)
for (var i=JStopmap.length-1;i>0;i--) {
// search for string of overlay layer
if (JStopmap[i].src!=null) {
// HACK -- berkeleymapper is a unique string of the WMS service i'm calling
// this can be anything.
if (JStopmap[i].src.indexOf('berkeleymapper') > 0) {
// browsers handle opacity differently, these settings should handle most common methods

4) Finally, I implemented a slider bar with some modifications in my script to link it up with the adjustOpacity function. This is from Mark Wilton-Jones library of JS functions
There were other slider scripts out there but this one seemed easiest to work with, though i don't care much for the colors and it is a bit involved to change the look of the control.

Wednesday, August 24, 2005

Fun with projections

Two error circles representing point certainty are displayed on the first map in geographic space. This rendering is from my first implementation of BerkeleyMapper which used geographic space for all maps.

The second map displays these same error radii in the second implementation of BerkeleyMapper which leverages GoogleMaps API. Google uses a Mercator projection, which preserves local shape but has a large degree of exaggeration towards the poles. Both of these samples are technically correct I suppose but I'm not so happy with the vertically exaggerated ellipse from the GoogleMaps API example.

The vertical exaggeration of the ellipse in the second sample, however, is preferable to the "squishing" affect of geographic space, particularly when underlaying USGS topos (as the map text will appear flattened in northern and southern latitudes).