HomeUpSearchMail
NEW

Google Earth and accurate skyline profiles

• I understand that GE uses WGS84 longitude/latitude. Although GE is using a simple map projection, the real errors are small for medium distance directions (I assume).
  
• GE is not using a real WGS84 geod to map the terrain imagines (it uses a approximation of a sphere by using several flat surfaces [oblate spheroid], like most 3D programs do).
  
  
  
• The height is refered to sea level (<altitudeMode>: tidal datum), thus not according to WGS84 ellipsoid but EGM96 geoid. Also here there will be some error due to sea level instead of geodetic datum, but in medium regions (up to 10 km) and not too large height difference (say less than 1 km) no real errors will occur.
  So I assume the eye alt (as seen in the status bar) determined from the tidal datum and not from the oblate spheroid surface.
  There is a difference between the 'eye alt. based on oblate spheroid' and 'eye alt based on tidal datum'. The 'eye alt. based on tidal datum' is closer to the correct one (which is 'eye-alt based on geodetic datum').
  
• A calculator to translate sea level height (based on EGM96 geoid) into WGS84 can be found here.
  
• There are known errors in the mapping of the terrain imagines to the grid. I have no ideas about these errors, so in some way I prefer to use my own GPS measurements instead of the 'picture of a site' on an terrain imagine. So in principle I don't use the terrain map for the measurements in this case (although they are nice for an overall ambiance of course), and it would be nice to get this in general accurate.
• 30 meters or so is the typical accuracy for placement of DigitalGlobe images.
  
• It looks there are parallax inaccuracies in Google earth (like between the photographic top and geometric top of Giza: 33m difference) and a position of a marker is depending on the zooming (can be some 10 m difference depending on zooming). Also the elevation top in Google Earth is some 110 m off from the GE photographic top. The difference between geodetically determine top of Khufu (using Helmert spheroid: coverted to WGS84: 29.979161 N, 31.134178 E) and GE geocentric top (using WS84) is less then 1m.
  See for an example here:
  
  View Larger Map
• The height resolution data used in the present GE is poor (tested upto version 4.2.0181.2634), even if we assume that a height step resolution of 1 m is oke for medium distance direction. This could be improved in GE by streaming more accurate height information when using the tilt slider.
• Due to the above, skyline profiles in GE are very poor (when using Tools -> Options -> 3D View -> Terrain Quality -> Higher):
  



Compare this with this one using a camera:

(my own VRML world provides a comparable picture as with the camera)
The heights of the hills are around 450 m, the eye alt is around 15 m and distance some 12 km.
• I have the idea that terrestrial refraction is not incorporated in the displaying of the scene when using tilt slider. I think it would not be difficult to include terrestrial refraction.
• It seems that having one's eye alt at say elev+2 m (so a person standing on the ground) is not possible, the tilt slider is then not really working.
  When tilt slider is fully at skyline view, eye alt can't be made smaller then a certain value (depending on the elev?), and this certain value is not near a person standing on the ground.
• There is no indication of the eye's direction (azimuth), it would be important to have the azimuth (with at least one decimal) also in the status bar when rotating the navigation ring
• It is important to see on the status bar also the eye's long and lat. Now one only sees these all for the mouse pointer.
• The status bar should also provide the azimuth and altitude angles of the mouse pointer with respect to the eye location.
• I think Google Earth has a good potential in the future, the nice thing is it looks to become more popular then VRML or X3D products and sometimes the sheer amount of viewers will determine the direction of the market and the developments. So I think investing some time in this can be important.
  
• So it would be great to have Google Earth providing an accurate skyline, I don't expect that the people of GE will look into the above ideas very soon, but one never knows.
  

Disclaimer and Copyright
HomeUpSearchMail