I'm afraid I've a very basic question about interpreting UTM grid values:

**Are the grid references in metres?**

I have data from the Spanish catastral map labelled as UTM Zone 31 ETRS89, and want to calculate some point-to-point distances, by pythagoras, over small distances used in urban planning.

If so, then over tens of metres we should expect extremely small errors from treating the ground as flat and simply using pythagoras, in order of 10-10 metre? (Obviously other errors will dominate!)

Reasoning:

* 2 x π x 40 metres / 40,000,000 metre circumference = 6 x 10-6 radian
* 6 x 10-6 - sin(6 x 10-6) = 4 x 10-17 (very possibly beyond accuracy of calculator)
* 4 x 10-17 x 6,300,000 metre radius = 2 x 10-10

This is what I calculated for a building:

```
372954.477 4316151.012
372961.247 4316155.121 dx   6.770 dy   4.110 =>  7.920 m
372971.827 4316161.551 dx  10.580 dy   6.430 => 12.380 m
372980.437 4316147.931 dx   8.610 dy -13.620 => 16.113 m
372963.327 4316137.601 dx -17.110 dy -10.329 => 19.986 m
372954.477 4316151.012 dx  -8.850 dy  13.410 => 16.067 m

```