Tuesday, January 17, 2006

US Survey Feet vs. International Feet

I get asked this now and then and I can never remember. This is not an issue for most people. If you live in the US, you are probably using US Survey Feet. If you're not in the US, you're most likely using meters. The only place I've ever seen International Feet used was Prince William County, who used that unit until about 2002, when they joined the rest of the country and started using US Survey Feet. Since that time, I've now and then run across a dataset that didn't get converted, or found a survey that's still in International Feet. In Virginia State Plane NAD83, you'll notice your data being offset by about 20 feet if you mix up the two units.

Here's a brief summary of how to convert from one unit to another:

1 International Foot = 0.3048 meters exactly
39.37 Inches (US Survey) = 1 meter exactly

To go from X US Survey Feet to Y International Feet:

Take X
Multiply times 12
Divide by 39.37
Divide by 0.3048 = Y
Overall conversion factor = 1.000002000004000008000016000032 (approximately)

To go from Y International Feet to X US Survey Feet:

Take Y
Multiply times 0.3048
Multiply times 39.37
Divide by 12 = X
Overall conversion factor = 0.999998 (exactly)
If you're in Autocad, you can scale all your features from 0,0 using the overall conversion factor. This assumes that whoever produced the drawing was using a coordinate system at all - a pretty big assumption at many firms.

If you're in ArcGIS, you can alter the projection properties so that the units are the correct one, and ArcGIS will do all the coordinate conversions on the fly for you.

2 comments:

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Dirtman said...

Thankfully, most of us use US Survey feet, but don't forget about South Carolina who are the Unions exception. I recently read a note from a set of plans from a Virginia based firm who conducted their topographic survey based on International Feet. If that was a typo, that is a VERY hard mistake to make! Who proofreads these plans?