NASA World Wind FAQ’s
The does not appear as it should be?
If the area is totally flat and in such a case you are looking at it below or above 60° latitude, it may be possible that there may be no elevation data. This is because the satellite may have taken the measurements, by orbiting just around the equator. Therefore the latitudes above 60° where too extreme for taking the measurements which resulted in non-decent quality data.
If it is not so then it may be because the elevation data, SRTM is acquired just the same way as bat's echolocation and sonar measurements. Thus, the elevation is “sounded” from a satellite. However, such soundings are only taken from 90 meters. Because of this the mountains may appear strange.
Also take a note that the satellite collects the elevation point in the form of a grid. Therefore, when it is taking the images, it is actually flying over between two adjacent mountain peaks. If the peaks are 50 meters away from each other, it may take one reading say 40 meters from the one peaks top. However, it may manage to hit the second pick right on the top. Thus, in this case it never hits the tippee top of the first peak. Because of this the elevation data is not that perfect.
One more problem, is that for some areas, which have high relief, soundings are not obtainable. Therefore, the data derived from SRTM is not all accurate. However, from the year 2006 Google Earth and NASA World Wind are trying to install improvements in this elevation data. For this, they are using data from other sources. There are some other sources which have more accurate 3 dimensional views for a number of mountains, which also include Matter horn and Everest.
For cities there are different problems that the mountains. As the soundings are taken in the form of grid, the elevation in cities having skyscrapers is so varied that the effect known as “mountainous spikes” is produced. In a city, just imagine, that one sounding may hit a tall skyscraper and the other may hit a car, the third may again hit a building or even ground. Therefore the elevation in cities appears like a big field consisting of randomly placed spikes.
When you are viewing a city, the simplest way for solving this problem is turning off the elevation. This you can do by simply pressing the “0” key.
Some surfaces appear bumpy at the points where they should not have been. This looks similar to airplane runways. The reason for this is that there are different materials and ground coverings. They reflect the radar in a wrong false way and therefore false readings are obtained.
With this information you may have got an idea about how the data is collected and therefore why the problems arise. However, you can get more information about the methods which used for collecting data, at the JPL Website.
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