Textures on private islands question

My current understanding is, that if you wish to change textures on a PI, you have to change the entire island. Am I correct ? I know that there are four different zones (northeast, northwest, southeast, southwest) for height elevations, but I see nothing as far as customizing each zone's textures.

I do not have an island as of yet. However I do think that your first assertion is correct
your combination of choices effects the entire island.

I have recently created some textures designed with sims in mind.

Feel free to IM me if your interested.

You are correct.

Terrain textures are specified sim-wide. You specify four textures for four "altitude bands", and you set what the transition points are at each of the 4 corners of the sim. The same 4 textures apply to the entire sim. The four corners adjust where the transitions are.

So for example, if you use the same values at each corner, you could use a coral or sea bottom texture for 19 M or lower; sand between 19 and 23; grass from 23 to 40; and rock with clumpy grass above 40. Nice sandy beaches all the way around, assuming a default sea level of 20 M. Now, if you changed the numbers at one corner to be 4 less, then in that corner the bands dip down, and you'd have grass all the way to the water line in that corner.

The edges between each band are handled by SL, which blurs the textures together over a 2 to 4 M altitude transition.

In the estate tools section where you see the 4 corners, by clicking on the textures shown you bring up the texture chooser. There are 4 textures for every island and you can only control at what height they begin to show at. Your "low" texture is the one that will start at the very lowest point of the sim and the "high" texture is what will show at the highest peak of the sim. You adjust the settings in each corner to suit your terrain. So, for example, if you've chosen lava rock as a mid-level terrain texture you can have that start in the NW corner at 25m (right above the water line) and not start until 40m in the NE corner. The textures blend for a bit as they transition from one to another, so you will have areas where your #1 texture bleeds into your #2 texture for about 5m.

Thanks both Ceera and Isablan for excellent answers.