[Bison] Mesh from GeoTiff has wrong elevation values?

Grasshopper
1

Hi guys,

I have been trying to use the new Bision plug-in to work with GeoTiffs and Meshes.

I downloaded a GeoTiff from OpenTopography and inputted into “Import Mesh DEM” component to generate a mesh from it. But the elevation is clearly exaggerated, I do not understand why… I scaled the mesh in Z just to make it look better, but I would like to have the real elevation as close as possible.

Mesh01
Mesh01.png1572×640 846 KB

I tend to think it has to do with the plug-in itself, as the GeoTiff is a standard one from OpenTopo…

Mesh02
Mesh02.png1572×640 429 KB

I also have doubts regarding the dimensions of the mesh, as the inputs seem to be in pixels…
I am not sure what values to input so that the mesh has the real extension in kilometers…
Sample: Sampling factor as nth pixel to read
XDimension: X dimension pixel size.
YDimension: Y dimension pixe size.

Hope someone can shed some light on these issues for me.
Unfortunately Tiff isn’t a supported/authorized extension, so I cant upload it, but if you need more info like the metadata of the GeoTiff I can provide it.

2

Hi - I’ve tested the GeoTiff import of Bison quite a bit and found it to be very reliable.
You need to find out the real world resolution of the image and use that together with your Rhino document units. For example, the datasets I use are sampled every 0.25 meters and I use all samples. That gives Sample set to 1 and X- and Y-Dimensions set to 0.25 in a Rhino file in meters.

image
image.png1526×706 1.84 MB

To check the elevation, I selected 1 point where the gumball is and checked that against the map and both are at 179 meters msl.
-wim

1 Like
3

Hi wim, thanks for the reply. According to Qgis, this is the metadata of my GeoTiff.

I didn’t quite get how you achieve the ideal values to input in Sample, and X and Y dimensions. Are you able to deduce those from my information or I am missing something?

Qgis%20data
Qgis%20data1212×824 175 KB

What do you mean exactly by against the map? All I have is a GeoTiff in grayscale, which I downloaded from OpenTopography by selecting a region.

Edit: (SRTM GL1) Global 30m Ellipsoidal(SRTM_GL1_Ellip) means it sampled every 30meters??

That would mean?
Sample 1:
X and Y dimension = 30
Rhino Units = Meters

4

Yes - provided you are using all pixels. If not, you can increase the Sample value.

FWIW, I checked what Qgis tells me about the GeoTIFFs I use.
For one of my datasets, Qgis reports this:

image
image1002×875 133 KB

… the interesting information being all the way at the bottom:

Pixel Size 0.25, -0.25

and further up:

Unit: meters

But I know that information when I pick the data source from my provider:

image
image372×840 22.9 KB

(the “(0.25 m)” in Asker 2012 (0.25 m).

Just that I checked against some reference that I know has the exact elevation of a point, in this case a map on the web:

image
image1091×296 92.6 KB

-wim

1 Like
5

Thank you for the detailed reply Wim! It is nice to compare workflows with someone else.

The units of my GeoTiff are: Degrees. Because it’s CRS is EPSG:4326 - WGS 84 - Geographic. I might need to project it to a CRS that supports SI units.

I am in doubt about pixel size now though. Mine says: 0.00027… , but the data is Global 30m Ellipsoidal.
Maybe I will just use 0.00027 as that is what Qgis says is the pixel size.

6

Yes, that corresponds with the 30 m or 1 arc-second.
1 arc-second is 0.0002777778 degrees.
So, a pixel is still 30 meters. Have you tried to use that in Bison and see how that comes out in Rhino?
-wim

1 Like
7

After 12min of calculation it worked! Thank you Wim.

success
success1583×631 644 KB

1 Like
8

Nice!
I see that you down-sampled to 3 - those GeoTIFF files sure lead to some rather large Rhino files… :sunglasses:
-wim

1 Like