Archive for February, 2008

YouTube layer in GoogleEarth

February 16, 2008 Leave a comment

Hills in Sullivan City AreaGoogle Earth has a layer that consists of YouTube videos, I realize this is old news but I have not had an opportunity to play with this layer until recently. I was really excited when I first started using this layer, as it has the potential to allow somebody to get a feel for an area. I realize that services such as Flickr and Panoramio already this sense of immersion but video would allow it to go one step further. Not only could someone see a particular hill, they could watch how fast someone could transverse the terrain. Not only could someone see how people look like in their local environment, they could see how they act, their manner of speech, their accent, lingo particular to an area, as well as other local characteristics. What a good example of human geography.

North of Rio Grande City

I tested a few geographical areas that I am somewhat familiar with and unfortunately most of the YouTube videos are incorrectly located. The following picture depicts a raceway, Six Flags, some concerts, people interacting in different settings and a few others. However the geography of the place is ranch-land with an arroyo (ravine) running nearby. There are no structures and there are no people.

I looked at the YouTube site for information on how they geotag their videos and the only thing I could find was that users supply the coordinates. This seems rather odd for those places that have multiple geotagged videos that are obviously in the wrong location. Perhaps they use a combination of user geotagged videos and some other automated geotagging algorithm for those videos that are are left blank. That is pure conjecture based on the results I saw in a few select areas.

I looked at other areas and these areas seems to have videos consistent with what I would expect for a given area and video that is inconsistent for certain types of environment. My limited experience with the incorrectly geotagged videos gives me some reservations about accepting all results at face value.


Scale and Scale Factor

February 4, 2008 2 comments

1:100,000 is a ratio, on a map it is referred to as map scale. The one represents a unit of measurement that represents 100,000 units of measurements in the real world. For example if I measure 1 inch on the map, I should be able to measure 100,000 inches on the earth for the same area.

The map scale is found by dividing the distance on the map by the distance in the real world, see this link. If I measure -100W by 40N to -100W by 35N on a map I get 2.77 cm. I then measure the real world distance using the NOAA’s ellipsoidal measurement tool to come up with 55,493,612.86 cm. By using these numbers in the equation I get the scale, 2.77cm ÷ 55,493,612.86cm = 1:20,000,000 (to get 1:20,000,000 exactly I would have had to measure the map as 2.774680643 cm). For this particular map 1:20,000,000 is the principle scale, ie the displayed scale.

However, map scale is not static, it is not the same everywhere on the map. It depends on the projection used and how the projection distorts the world. In an equidistant projection the scale is correct between certain points. The equidistant conic projection‘s scale is correct for the meridians and the standard parallels. So in the example above, the map was projected with the World Equidistant Projection and the principle scale was 1:20,000,00 as shown. If I move to a new location and measure -105W,30N to -100W,30N on a map I come up with 2.6 cm. I then measure the real distance on the earth using NOAA’s tool to and come up with 48,239,311.01 cm. Putting these numbers into the scale equation yields; 2.6 cm ÷ 48,239,311.01 cm = 1:18,555,581. This scale was taken from a location that was NOT along the standard parallels or along the meridians, it is referred to as as local scale.
To find the scale factor we divide local scale by principle scale. Using our examples above 1:18,555,581 ÷ 1:20,000,000 = 1.077959. Or in other words the local scale has been exaggerated by 107% This is all rather confusing and actually makes my head hurt, a really good explanation of all this is found in Portraits of the Earth: A Mathematician Looks at Maps by Timothy G Freeman ISBN 0821832557, chapter 3.

Scale and Scale Factor

Categories: GIS Tags: , ,