The linear unit of measure of a projected coordinate system should be the simplest part, but choosing the wrong unit can cause problems with data. Any linear map projection parameters, like false easting and false northing, use the same linear unit as the projected coordinate system definition. The false easting and false northing values can be in the millions, so a mistake in the unit of the coordinate system can be magnified. One common mistake is choosing a linear unit that uses the wrong foot-to-meter conversion value. There are two foot-to-meter conversions in common use. The "Foot" definition is the international foot. In the United States, the survey foot is also used.

Another common problem occurs when the linear unit is assumed to be one of the foot definitions when it should be meters, or vice versa. This problem is easier to identify, because data will usually not overlay with other data. The offsets will often be hundreds of thousands of units. In the earlier problem, where the foot definition is incorrect, the offsets are usually a few values off.