"I have discovered that despite the defined projection (wkid), extents are returned in meter coordinates with an origin of lat/lon 0,0 (off the coast of Africa)."
With ArcGIS Javascript API, the first map service added sets coordinate system for the whole map. If you added one of ESRI's tiled map services for a basemap, it's probably one that was created using wkid 102100, which fits what you described. When you lay your map service on top of it, your services is reprojected to fit on top of this service. To fix this, you either need to create your own tiled map service basemap projected in the wkid you want, or use the geometry service to reproject points, as you have done.
"I use a Geometry service to project them, however when the user pans around the world, the coordinates do not reset, they just keep adding up, resulting in 'NaN' extents returned."
Here's a function you could use to fix the extent to something you can use for your geometry service:
function fixExtent(extent) {
// hard-coded full extent of the world, using wkid 102100.
var full = new esri.geometry.Extent(-20037508.3427892430765884088807,-19971868.8804086,20037508.3427892430765884088807,19971868.8804086, new esri.SpatialReference({ wkid:102100 }));
// calculate the extent size so that it fits within the normal extent
var xwidth = full.xmax - full.xmin;
var newXMin = extent.xmin;
while (newXMin < full.xmin) {
newXMin += xwidth;
}
while (newXMin > full.xmax) {
newXMin -= xwidth;
}
var newXMax = extent.xmin;
while (newXMax < full.xmin) {
newXMax += xwidth;
}
while (newXMax > full.xmax) {
newXMax -= xwidth;
}
// return a new geometry that is within the range of the full extent
return new esri.geometry.Extent(newXMin, extent.ymin, newXMax, extent.ymax, extent.spatialReference);
}
I don't know how this will work if you compare an extent that crosses the dateline. Hope this helps.
Best Answer
When you create map object, you can specify the extends as below:
Then latter you can change the map extends values as below:
http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi_start.htm#jsapi/map.htm#setExtent