Am I missing some step in making maps?

1: terrain.party, save a section

2: brighten a little to differentiate the sea from the edge.

3: cap the brightness so the mountains are not too tall.

4: save image with 72x72 px/in resolution at a size of 8192x2048px.

5: flip image.

6: save in mod folder as map.data

Then I come out with: https://imgur.com/a/Y4UAdgq

I fooled around with the greyscale, and found this:

1. the map is sea at brightness 0-14 or so,

2: then produces 1 meter tall land at 15-29,

3: then has impossible tall mesa at 30-100