[GIS] converting UTM – Decimal Degrees with javaScript or using a web service

javascriptutmweb service

I am creating a wildlife survey form that has a field where users enter location. Users will be able to enter location using a map (i.e. draggable pushpin) or by manually entering lat/lng. My clients are also requesting an option for entering UTM coordinates.

If a user does enter UTM coords I would like to convert these before pushing to the database. I've seen a couple websites that have utilities for converting UTM to decimal degrees.

Does anyone know of a javaScript lib that will make these conversion given zone and easting-northing? I also saw this thread

Convert geographic coordinates to UTM zone 29N with Javascript

Does ESRI have a web service that I could pass UTM coords to and get lat/lng back?

Best Answer

If you're not happy with using a Geometry Service like Kirk suggested, you could use proj4js client-side.

Related Solutions

[GIS] Determine the UTM zone/hemisphere in a particular datum

The choice of datum doesn't matter for map making (provided you change datums appropriately, of course). It does matter for transmitting coordinates and sharing data.

To address your last question ("does it even make sense?"), note that UTM is really a coordinate system and as such--in addition to its grid-based zone naming system--it includes a definite datum and unit of measurement as well as a set of projections. What we're talking about here is using its projections (Transverse Mercator (TM) for the low to moderate latitude zones) and its zoning system but possibly changing the datum (and, perhaps, units of measurement). Changing either the datum or the units (or both) is fine, but after that is done the result is not "UTM," strictly speaking. In some applications (e.g., military targeting) it could be dangerously misleading to refer to the resulting (x,y) values as "UTM coordinates," because they will look remarkably like UTM coordinates but might be up to a few hundred meters off. (That can lead to embarrassing things like lost embassies.) Nevertheless, because your principal questions refer to "UTM" in this looser sense, I will also use the term in this way.

UTM is a choice: you can pick any zone to use. It is designed to work within certain accuracy constraints to within 3.5 degrees (longitude) of its meridian at any latitude within the 120 non-polar zones. Because the zones are nominally 6 degrees wide, this allows a 0.5+0.5 = 1 degree longitude overlap with each neighboring zone. Moreover (if you are concerned with map quality foremost and following the UTM system exactly is of lesser importance), the overlap can get larger in the more extreme latitudes (e.g., at 45 degrees the overlap is more than 4 degrees of longitude). Because two (reasonable) datums differ in longitude by at most by 100 meters or so (0.001 degrees), the difference in datum is inconsequential: if you choose a UTM zone based on one datum it will work fine with another.

Once you have allowed yourself the flexibility of changing the datum, you might as well go all the way and create a "custom UTM zone." This is tantamount to dropping UTM altogether (which includes forgetting about the 0.9996 scale factor and origin offsets) and selecting a TM projection suited for the area you are mapping. Within a small area you can do well by placing the meridian of the TM projection through the center of your map and using a scale factor of 1.0000. Choose any convenient origin.

[GIS] Draw a point with ArcGIS Javascript API

Adding a point from a button click can be as simple as:

dojo.connect(dojo.byId('pointButton'), 'onclick', function() {
  map.graphics.add(new esri.Graphic(
    // Point coordinates are 0, 0
    new esri.geometry.Point(0, 0, map.spatialReference),
    new esri.symbol.SimpleMarkerSymbol()
  ));
});

And the pointButton node is:

<button id="pointButton">Click to add a point.</button>

To add a point when the map is clicked is pretty straightforward as well:

dojo.connect(map, 'onClick', function(evt) {
  map.graphics.add(new esri.Graphic(
    evt.mapPoint,
    new esri.symbol.SimpleMarkerSymbol()
  ));
});

Live example showing both here: http://jsfiddle.net/swingley/auyHf/

Regarding converting from lat, long to another coordinate system: if you're converting to or from lat, long to web mercator, there are client side methods available to do this: esri.geometry.geographicToWebMercator and esri.geometry.WebMercatorToGeographic. If you need to convert from lat, long to something other than web mercator, then the geometry service's project method is the way to go.

Best Answer

Related Solutions

[GIS] Determine the UTM zone/hemisphere in a particular datum

[GIS] Draw a point with ArcGIS Javascript API

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