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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Layer Files
Layer files are based on raw data files. Basically, a layer file tells ArcMap how to draw a data file—what symbols and colors to use. As you know, if you add a raw data file in ArcMap, the software makes random choices as to how feature are drawn. Sometimes this is satisfactory; more often it is not, if you have serious intentions of examining and analyzing the data. Let’s look at an example, in which you can see an obvious advantage to choosing how features are symbolized.
____ 1. Use ArcCatalog to copy the shapefile KY_Streams_spf from
[___]IGIS-Arc\Kentucky_wide_data
to
___IGIS-Arc_YourInitials\Map_Making.
____ 2. Start ArcMap with a Blank Map. Add
___IGIS-Arc_YourInitials\Map_Making\KY_Streams_spf.shp
to the map, using Data View
This is a fairly large dataset (about 60 megabytes) that contains information about the streams of Kentucky, from the largest (Order 8) to the smallest (Order 1). When two streams of the same order (e.g., Order 1) come together, they make a stream of the next highest order (i.e., Order 2). However, if two streams of different order come together (e.g., Order 6 and Order 5), the output is just a stream of the higher order (i.e., Order 6).13 Therefore, “stream order” cannot be considered true ordinal data, in terms of stream size, volume, rate of flow, and so on, even relative to those streams that flow into it. That is, the Order 5 stream mentioned previously might have a greater flow volume than the Order 6 stream it converges with. ...

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