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Real world Line widths and color symbology

October 11 2008 | 0 comments
Categories: Symbology

Hi,

Its me again, (I asked the original post on the line widths.) and since you were so helpful before - here is another question relating to the line widths.

I now have the same line layer with all the streets and thier widths. I need to create a map using their real life width (meters and not just points) and using other attibutes (in this case: If the street exist or planned, its statutory framework... basically three different fields - attributes - which will give me 7 different line categories each of a different color.) I tried using "symbology - multiple attributes - quantity by category - and varing the symbol size based on my field with the line widths. But this only give me a width based on points and not real life widths. How can I use attributes of the fields for the color and the street width for the width of the line, all at one time? I thought of creating selection sets and then copying them to a new layer and then setting the width - but it seems like way to much work for something that (I think!) should be relatively simple.

Hope you can help...

Mapping Center Answer:

First, I would simplify the information in your three fields--you indicated having only 7 categories overall.  Add a new field to your data.  The query your attributes, selecting each unique combination and as you do so calculate the value of the new field to be something to represent that combination (if you're using a geodatabase you could use an integer field and then set up a coded value domain to manage a user-friendly description of what each integer represents).

Then you could more easily use the Quantity by Category where the categories are the values from the new field, and the quantities are from width field.  The trick to getting the widths to work is once you click the Symbol Size button on the symbology tab, choose your value field, and then click Classify  You will want to put a class break between each value on the histogram. 

You will still need to figure out a conversion factor for points to real world units.  So, assuming a point = 1/72 of an inch, you can use your map scale to figure out symbol sizes. 

For instance on a 1:4,800 scale map, a 30-foot-wide road would would need to be 0.00625 feet, which is 0.075 inches or 5.4 points. The math is:  30/4800 * 12 * 72.  Just replace 30 with each specific street width and the scale with your map scale (this presumes your measurements are in feet).

I typically make a spreadsheet where I can just copy the formula for each street width.  Then you will have to manually set the sizes of each symbol.  If you set a reference scale, you can change map scale and have the results work at other scales in that map.

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