The hypsometric contour interval on the classic USGS quadrangle maps is 40 feet. Although it’s not a nice round number, I think it looks fantastic at that scale. It’s good at representing many different terrain types. Which is why USGS contours have become a bit of a standard for many GIS and cartographic applications.
But what if you are mapping at a much larger scale? For a county-sized area, 200 feet contours may be more appropriate. Unfortunately, it’s not always trivial to recontour the original surface, especially if the data is not readily available.
The good news is that you can easily extract the 200 feet contours from the 40 feet contour dataset. The key to this process is Select by Attributes in ArcGIS. A contour dataset is really just a set of polygons with an elevation field (named ‘ELEVFT’ in my sample dataset). Selecting only the contours that are evenly divisible by 200 requires the modulo function. The modulo function takes two numbers, divides them, and then returns the remainder. If the remainder is zero, that means the two numbers divide evenly. In ArcGIS SQL syntax, this is what your selection statement looks like:
MOD(“ELEVFT”,200) = 0
Once the contours have been selected, it’s easy to create a new shapefile and add it to the map. You can also extract the 1000 feet contours, and plot them as index contours. I personally like to use 1pt black lines for contours, and 1.5pt black lines for index contours. On a busy map, I also like to reduce the contour layers to 60% transparency.
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