[OpenLayers-Users] ArcGIS Online Cached tiles datum shift

[Vadim Bokin]

Robert,

I've made good progress with your suggestion.  I created a new class
and just extended XYZ.js with the 2 methods you provided.  But I still
have a problem.

Old Code: locations where close, but kept jumping around north or
south of the right location

New code: locations are almost exactly 41.0 miles North of where they
should be. But they are off by a fixed amount.  So when I zoom in and
out they are staying put where they are, but they are still off.  So
if I try to put a point on the beach in Santa Monica, on the map it
looks to be between Santa Clarita and Palmdale.   As soon as I change
base maps to Google or a epsg:4326 map, all is well and points are in
perfect location.

So it's 1 step forward (locations don't jump) and 1 step back
(locations are pretty far north).

I read tileOrigin from the AGS layer spec, e.g.
http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer.
 And I've made sure it is correct by testing in firebug and just
dumping out map.baseLayer.tileOrigin.

Did you do anything else to modify XYZ to make this work?  Does 41.0
miles mean anything?  to me it doesn't look like 1 degree, or anything
else.  But it's a fixed distance difference between plotted and viewed
points.  Some kind of error in calculation or my setup of the layer?

Thank you!

-Vadim


On Mon, Jul 19, 2010 at 6:24 PM, Robert Sanson
<Robert.Sanson at asurequality.com> wrote:
> Hi Vadim
>
> What layer type are you using? ArcGIS tilecaches have a tile origin in the
> top left. The standard layer.XYZ in OpenLayers is built to support a lower
> left tile origin. You need a patched XYZ.js with setMap and
> calculateGridLayout copied from Zoomify.js:
>
> * APIMethod: setMap
>      * When the layer is added to a map, then we can fetch our origin
>      *    (if we don't have one.)
>      *
>      * Parameters:
>      * map - {<OpenLayers.Map>}
>      */
>     setMap: function(map) {
>         OpenLayers.Layer.Grid.prototype.setMap.apply(this, arguments);
>         if (!this.tileOrigin) {
>             this.tileOrigin = new OpenLayers.LonLat(this.maxExtent.left,
>                                                 //this.maxExtent.bottom);
>                                                 this.map.maxExtent.top);
> //patched to be similar to Zoomify
>         }
>     },
>
>     /**
>      * Method: calculateGridLayout //patched from Zoomify
>      * Generate parameters for the grid layout. This
>      *
>      * Parameters:
>      * bounds - {<OpenLayers.Bound>}
>      * extent - {<OpenLayers.Bounds>}
>      * resolution - {Number}
>      *
>      * Returns:
>      * Object containing properties tilelon, tilelat, tileoffsetlat,
>      * tileoffsetlat, tileoffsetx, tileoffsety
>      */
>     calculateGridLayout: function(bounds, extent, resolution) {
>         var tilelon = resolution * this.tileSize.w;
>         var tilelat = resolution * this.tileSize.h;
>
>         var offsetlon = bounds.left - extent.left;
>         var tilecol = Math.floor(offsetlon/tilelon) - this.buffer;
>         var tilecolremain = offsetlon/tilelon - tilecol;
>         var tileoffsetx = -tilecolremain * this.tileSize.w;
>         var tileoffsetlon = extent.left + tilecol * tilelon;
>
>         var offsetlat = extent.top - bounds.top + tilelat;
>         var tilerow = Math.floor(offsetlat/tilelat) - this.buffer;
>         var tilerowremain = tilerow - offsetlat/tilelat;
>         var tileoffsety = tilerowremain * this.tileSize.h;
>         var tileoffsetlat = extent.top - tilelat*tilerow;
>
>         return {
>           tilelon: tilelon, tilelat: tilelat,
>           tileoffsetlon: tileoffsetlon, tileoffsetlat: tileoffsetlat,
>           tileoffsetx: tileoffsetx, tileoffsety: tileoffsety
>         };
>     },
>
> Using the above works perfectly.
>
> regards,
>
> Robert Sanson
>
>>>> Vadim Bokin <vbokin at gmail.com> 20/07/2010 11:53 a.m. >>>
>
> Hi,
>
> Does anyone have the _exact_ proj4js definition for ESRI projections
> EPSG:102100 and the older EPSG:102113?
>
> My maps have a y-coordinate shift (both north and south at different zooms)
> off by many map pixels.  Always close, but no cigar. X-coordinate is always
> dead on.
>
> I can add in ArcGIS online content as dynamic layer without any problems.
> But when adding cached tiles, there's always this shift.  Has anyone seen
> this before and/or been able to resolve it?  I need to be able to use the
> cached tiles.
>
> I've used the one from spatialreference.org, as well as set it directly to
> be equal to Google's EPSG:900913 but there's always a shift.  I've tried
> many different variations with  +a and +rf and others but I feel like the
> definition is missing a +towgs84=...  adjustment to properly correct for the
> shift on the map but I have no way of knowing what those parameters should
> be and whether it should be the 3-parameter adjustment or the 7-parameter
> one.
>
> Here's my current definition:
>
> Proj4js.defs["EPSG:102100"] = "+proj=merc +lon_0=0 +x_0=0 +y_0=0 +a=6378137
> +b=6378137  +units=m +nadgrids=@null";
>
> +towgs84=0,0,0,0,0,0,0  doesn't help, but I feel like the right values in
> there will fix the problem.
>
> Thank you,
>
> -Vadim
>
>
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