I am learning about UTM coordinate system and reading in Wikipedia:
In any zone a point that has an easting of 400000 meters is about 100 km west of the central meridian. For most such points, the true distance would be slightly more than 100 km as measured on the surface of the Earth because of the distortion of the projection.
As I understand, it comes from the fact that UTM uses not tangent but secant cylinder projection. By the convention, it maintains scale factor of 0.9996 at central meridian, which means the cylinder intersect the globe at approximately 180 km in both directions from the central meridian forming two standard lines for a zone. Due to this, the scale between standard lines is slightly less than 1, and is greater than 1 beyond them. As it is only 100 km, it lies entirely between standard lines.
(Scale is not followed.)
I understand that Mercator projection is not a straight line from the centre, but at small angles it should be very close to that.
Now I am trying to validate the statement from Wikipedia using the method below.
- I choose a point on the globe on the central meridian of a zone. For example, 72N, 117E (or 72,-117).
- Using the online converter, I get UTM coordinates. In my case, 500000 easting and 7988932.503 northing in zone 11.
- I change easting by 100 km, making it 400000 as in Wikipedia example (northing is kept the same).
- Using the converter, I get new lat/long coordinates: 71.978441,-119.897001.
Now I want to validate the statement from Wikipedia. I use an online distance calculator (I tried a few and they all seem to give the same answer, but this one gives meter precision). The answer I get is 99621.08 m.
I checked the above at different locations relative to the central meridian, and I consistently get the real distance shorter than 100 km (that can be inferred from UTM coordinates delta). Why does my result contradict to the statement and to common sense? For areas beyond standard lines it is understood, but why is it such between standard lines?