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Acquiring GIS Data: Utah |
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Scale:
You will also notice, from the SGID folders ( ftp://ftp.agrc.state.ut.us/sgid/ ), that several of the directories have numeric values associated with them, for example 024, 100, 250 and 500. These values represent the scale of the datasets in that directory. The scale is the ratio of map units to ground units. 1 map unit = 24000 units on the ground. This can be read as 1 inch on the map is equal to 24,000 inches on the ground. Units can be in any measurement as long as they are the same on either side of the comparison.
The following is an example of the most common scales used at AGRC:
The larger the denominator the smaller the overall number, thus 1/500,000 is a smaller number that 1/24,000. The larger the denominator is the more ground area is covered, yet less detail can be extracted. In contrast the smaller the denominator is the less ground area is covered, yet more detail can be extracted. If we remember the previous example, at a large scale (i.e. 1:24,000) a house might be represented as a polygon (area), yet at a smaller scale (i.e. 1:250,000) that same house will more than likely be represented by a point or not shown at all.
In the following example there are three different scales of Cedar City Utah. The area they cover on the ground is represented by the rectangles in the top image. The details that can be extracted at each scale are shown in the three bottom images. Notice that at 1:250,000 Cedar City is just a general outline, and as the scale increases to 1:100,000 you can see roads within the city and at 1:24,000 you can actually see some buildings.
Results ( scale_24_100_250 )
Use the following images to get a closer look at the details at each scale. Each image covers the same amount of ground cover. Notice how items become more generalized as the scale get smaller and how more detail is available as the scale get larger.
Results ( Cedar City 24k ) Results ( Cedar City 100k ) Results ( Cedar City 250k )
Questions: What scale of data would you use for a large ground area with not much detail?
What scale would you use that requires great detail for a small project area?
What would you need to do if you need a large area with great detail?
From the above image(s) compare the details at each scale. What details can be seen at the 100k level that can not be seen at 250k?
What details can be seen at the 24k level and not seen at the 100k level? |
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