Creating contour lines of different weights at 20' intervals, 100' intervals

Question

USGS topo maps have different line weights for contours at 20' and 100' multiples. I can use the Spatial Analyst contour tool to create contours at 20' intervals and again at 100' intervals and display them at different weights. Is there a way to do this with a single layer by somehow selecting the multiples of 100' and assigning them a different symbology? Alternatively, can I import the layer twice and use a query to select only the 100' multiples? I don't see how to do this? Perhaps I have to compute a new column that is true if it's a multiple of 100' and false if not?

GünterDörffel · Answer

Hi David,
 
that was probably a little misleading,
 
the computed field is the way to go ... I do not have a magic trick to do something like labeling classes for symbols. But a simplifying approach - if you need that more than just once - would be to create something like a "lookup-table" - say
 
SYMBOL   IsCONTOUR
1               20
1               40
1               60
1               80
2              100 
1              120
...
 
and then join this via IsCONTOUR to the CONTOUR-Field of your lines and then use SYMBOL for symbolization ... that way you wouldnt have to do calculates and create new fields all the time ...
 
Regards
Guenter

DavidStrip · Answer

actually, you don't have to go to the trouble of using contour list , contour already creates a field with the contour value, so your expression can be performed using plain vanilla contour.

But prompted by your comment, I figured out how to set the symbology based on the value of a computed column which is 1 for 100' contours, 0 otherwise. How do you use a formula to pick symbology classes?

GünterDörffel · Answer

Hi David,
 
I wonder: Did you see the "Contour List" Tool ... I think part of ArcGIS since 9.3?
 
Here you can specify for which values to create contours and then you have an attribute "Contour" that you can use to symbolize whatever you want ...
Like creating a special symbolization field ...
 
i.e you could use a query definition to select all contours that are 20' but not 100' with 
 MOD( "Contour",20) = 0 AND NOT MOD( "Contour",100) = 0

Give it a try
 
Regards
Guenter