1

I have the OSgeo4w installed on my PC and I am trying to upload ITM(israel transverse mercator) landmarks to google earth.
To do this I am using the cs2cs command.
I noticed that i get a deviation using the next code:
cs2cs +proj=longlat +ellps=WGS84 +datum=WGS84 +to +proj=tmerc +lat_0=31.73439361111111 +lon_0=35.20451694444445 +k=1.000007 +x_0=219529.584 +y_0=626907.39 +ellps=GRS80 +units=m
I tried the next code:
cs2cs +init=epsg:4326 +to +init=EPSG:2039
this time my deviation decreased significantly.
why does this happen?
Another thing is, when i use the EPSG code I get a third number (that is always zero using the first code).
The third number, after the lon and lat is around 20, it cannot be the elevation so what does it represent?

2

You're using the ellipsoid grs80 which is a general ellipsoid applicable for the whole world.

Each country have a set of parameters to localize their ellipsoid of choice for their region. That set of parameters are called datum, and they define how much an ellipsoid must be shifted at the x,y and z axis so to be locally correct.

You haven't specified the datum shift in your proj4 string. You need to add the +towgs84=-48,55,52 flag in your string.

so the string should become :

 +proj=longlat +ellps=WGS84 +datum=WGS84 +to +proj=tmerc +lat_0=31.73439361111111 +lon_0=35.20451694444445 +k=1.000007 +x_0=219529.584 +y_0=626907.39 +ellps=GRS80 +units=m +towgs84=-48,55,52

Using the flag -init=EPSG:2039 is like a shortcut to that lengthy string yout tried to type manually .

0

http://spatialreference.org/ref/epsg/2039/ gives the following for proj4

+proj=tmerc +lat_0=31.73439361111111 +lon_0=35.20451694444445 +k=1.0000067 +x_0=219529.584 +y_0=626907.39 +ellps=GRS80 +towgs84=-48,55,52,0,0,0,0 +units=m +no_defs

the tiny difference in scale may make a difference depending on how far from the centre of the projection you are, though in Israel I can't imagine you could get far enough from the centre to see a large error. How big is the error? Don't forget the the imagery in GE isn't always projected exactly, I've seen roads with "fault lines" running across them shifting the carriageway many metres.

  • Thank you, it is hard to evaluate the devation between google earth and the projection, it seems to me around 90 m. but the difference between the two projections i tried is 77 meters, at least for a landmark i just checked – eliavs Oct 16 '13 at 12:24

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