real terrain data with bathymetry

General Discussion
1

hi, new so please bare with me, I have an part of an island just off a piece of mainland with an area of seabed inbetween, this is real world terrain, so my question, how do I use negative values for the seabed as all the data seems to come slightly raised as if its the top part of a box, if I change the lowest value, its the bottom of the said box that has this, and not the lowest part of the sea bedm how do I solve thism I dont want to offset it, as I will be outputting to terragen and want to keep the real terrain heights
thanks in advance

2

Hi there,

I’m not 100% sure I’m following your question, but if I am correct, I think all you need to do is go to Project Settings -> General Setup tab, and adjust the Base Elevation to be below zero to account for the seafloor area. If your input terrain spans -1000m to 1600m, for example, then setup the elevation range to that (or a bit more than that) so that you can import your file unclipped.

Does that fix it?

3

thanks for the reply, had done that to begin with, but does not sort the problem, have attached an image to try to explain
thanks

4

what file format are you bringing your terrain in with?

5

its a 16 bit tiff, from 3d s max, colour range is from black to white, black being the lowest, so i thought that if i set the high and low values, which I know from global mapper, then it should be correct

6

OK, that helps!

Regular TIFF files don’t carry elevation information, so WM has to do some manipulations to achieve the right result. Here’s the two most likely possibilities:

  1. Your TIFF may not span the entire entire range. In your File Input device, in the Altitude Scaling area, what does it say for Elevation Span? It should be 0.00 to 1.00 if it is a fully normalized file. If not, try changing the scaling to “Specific Elevation Range” – you will need to know the exact min/max elevations of your terrain from global mapper that you are trying to match it to. Or, you could set to “Full Range” which will force the elevations to go to the full min / max values. In that case, make sure your project elevations are set to what you want your actual min and max should be, and you should be good to go.

  2. You might have missing data in a few spots that global mapper is exporting as a zero, throwing off the altitude span calculations. This is a nastier situation as you can no longer easily or automatically import the elevations. You can try changing the file input altitude scaling to “Specific Elevation Range”, and then proceed to set a minimum that is below zero, dropping the “floor” of the input terrain until your known heights reside where they should be.

Cheers

7

thank you for all your help, will look at what you have said and post results, many thanks

8

ok i think i have this nearly correct now, just trying to work out in my head how this works :),
values from Global mapper :
the lowest point of the sea bed is -16m
the highest point of land is 57m
in WM project settings in the general setup tab i set the base elevation to 16m and the max elevation to 57m+16+16mm= 89m this is presuming terrain altitude start at 0

in the file input, elevation span i put in 0m to 73m, which equates to the full elevation span

this looks right as far as exporting to terragen, image attached, and the range should be correct , but in layout view its giving me wrong altitude readings, the height is correct, but i would assume the sea bed would start from 16m( which would be ok, as its not relevant in this case), i just want the tile to sit at the right altitude in terragen, with the correct range, but the lowest depth reading is about 25m, confused :slight_smile:

also If I lower the base elevation the whole tile starts rising again, so I am a bit confused because i cant see why raising the base elevation lowers the tile

thanks for your help, i can also attach the tiff if you want to have a look at it, or provide a link, it 96meg

9

Incidentally, WM supports negative height base values: you can just set your low point to -16m and high point to 57m and be OK.

with regards to why lowering the base makes things raise:

here’s a simple thought experiment: If you set your base elevation to -2000m, all of your heights are at least 2000m above the lowest allowed value; thus they will appear to be lifted to near the top of the terrain cube. However, their actual elevations are still correct, WM is just showing you that there is room in the heightfield for thousands of meters of data below them. If your elevation value equals the base of the project elevation range, it will always appear to be at the very bottom of the terrain cube.

If you’re confused as to where the lowest value resides in the terrain, here’s a quick and easy way to do it: Create a Height Selector, and attach the terrain to its input. Then open it up and move the sliders to the very bottom of the height range, and reduce the falloff down to something low like 0.008. Now build your world. The white area of the height selector will show you what part of the terrain matches that height value. If you inspect your original terrain in the area shown to you by the height selector, you should see the same elevation value.

10

thanks for the detailed info, not had chance to look over it yet, but will let you know how i get on, thanks