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Referring Address Q & A

I’ll look through my list of "referring addresses" every now and then to get a sense of how folks are getting to this blog.  Often they are the result of some very interesting web searches.  Occasionally I’ll see a search and get a pretty good idea of what kind of information whoever was looking for; I wish I could hit "reply" to it to answer the question directly, but I can’t.  So every so often I will post some of the queries that landed folks to this blog and try to answer them specifically.

(query) different GeoRSS formats
(referring address) http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&q=different+GeoRSS+formats&btnG=Search

There are three formats: W3C, Simple, and GML. http://www.georss.org/index.html is a great resource.

can only describe point features, making it pretty lame.  Looks like this:


Simple is the most commonly used format and can describe point, line, and polygon geometry.  Points, lines, and polygons look like this (respectively):

<georss:point>47.236 -71.92</georss:point>

<georss:line>45.256 -110.45 43.84 -109.86 </georss:line>

45.256 -110.45 46.46 -109.48 43.84 -109.86 45.256 -110.45

GML is the most verbose and can handle complex geometry, like donut holes.  It is much less common and many APIs won’t support it.  Points, lines, and polygons look like this (respectively):

            <gml:pos>33.163 -64.89</gml:pos>

         33.216 -120.71 32.49 -119.51 31.66 -118.41

            45.256 -110.45 46.46 -109.48 43.84 -109.86 45.256 -110.45


(query) virtual earth 3d nad27
(referring address) http://www.google.com/search?q=virtual+earth+3d+nad27&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

NAD 27 refers to North American Datum 1927 (common for GIS data in the North American area).  A datum is the reference ellipsoid assumed for the almost-spherical shape for the earth.  It is important that you always know what datum your coordinates are assuming, or your points may not line up with their intended location on the map.  The big API players out there (Microsoft Virtual Earth, Google…) assume coordinates use the datum WGS 1984 (good for data/applications with a global coverage).


(query) platte carre projection

Platte Carre projection is a geometric transformation or our round earth that squeezes the lat long lines into an evenly spaced grid.  This projection is typically of desktop GIS programs and is the ubiquitous default used by ESRI.  It is commonly misunderstood as "unprojected", though that is impossible.  Most publicly available WMS layers use this projection, while Microsoft, Google, Yahoo, and everyone else uses Mercator.  Other names for it are Cartesian, Latitude Longitude (though this is actually a coordinate system, not a projection), or WGS84 (which is a datum, not a projection).  One downside of this projection is that things get stretched horizontally the farther you get away from the equator -especially obvious in aerial imagery in high latitudes.  It looks like this…


(query) renaissance mercator projection
(referring address) http://www.google.com/search?hl=en&safe=off&q=renaissance+mercator+projection&btnG=Search

Another quest for projection information, I see a lot of that.  Mercator is a projection that maintains real-world direction and shape, but distorts the relative size of things.  The maintenance of shape is a big deal for composite applications because aerial imagery is usually pretty important and seeing it all squished is distracting.  It also lends itself well to the notion of tiled mapping.  It is a very old projection but has found a huge new surge of popularity because of online map services.  It looks like this…

(query) Hillshade for Southern Hemisphere
(referring address) http://www.google.com/search?q=Hillshade+for+Southern+Hemisphere&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7SUNA

This might be referring to the relatively recent addition of hillshading to Virtual Earth map tiles.  Hillshading is a method of illustrating shadows over a map surface to indicate terrain variations with the goal of indicating some sort of 3-dimensionality.  It is accomplished assuming an imaginary light source (usually from the northwest) combined with a Digital Elevation Model and a line-of-sight algorithm.  Virtual Earth provides it as a parameter in the map call: VEMapStyle.Shaded.

John Nelson / IDV Solutions / john.nelson@idvsolutions.com



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