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I am exporting a 3D model in .obj format.I want to georeference it.

I require the export to be in the Global Coordinates System (UTM) The problem is that I am getting coordinates like:- v 0 0 -0 v 0 -145 0 v 147 123 125.3 v 120 -2.5 30 …

But I require coordinates to be like this (The results I got with other softwares georefrenced .obj data) v 733711.541 24.000 -3157883.355 v 733715.789 24.000 -3157880.456 v 733721.676 24.000 -3157888.570 v 733720.655 24.000 -3157889.166 …

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2 Answers 2

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This is a very specific task, if I may add a personal note to that. But luckyly I studied physics and happen to know the answer.

OBJ are saved in form of Vectors. One point is represented by 3 vectors along X,Y,Z -axis. Most likely they are called a,b,c. 0,0,0 being the origin. I don't want to go to deep into that. Unless you want that - let me now. Representation in Form of a,b,c, in a 3D space is called a Cartesian System. Since earth (approx) is a sphere we need to transform this Cartesian System into a Spherical System. From a,b,c, to Spherical coordinates (r, θ, φ) radial distance r, azimuthal angle θ, and polar angle φ.

you can calculate each point with this:

Cartesian (x,y,z) → Spherical (r,θ,ϕ) r=√x2+y2+z2 θ=tan−1yx ϕ=tan−1√x2+y2z

Last step is to calculate Sperical Coordinate to UTM which is a cylindrical projection. Which results in X and Y [Because a cylinder is a unfold rectangle] X=rθ y=r ln[tan(pi/4 + ϕ/2)] = r/2 ln [(1+sin(ϕ)/(1-sin(ϕ))]

There are calculators for this. search for Cartesian to UTM / Spherical to UTM

Hope it helps

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Its not crazy complex... you need to transform your model into UTM coordinates. Firstly you need a few known points on your model, for which you know the utm coordinates. The (Helmert) transformation can be done based in these corresponding points. If you want a quick and easy way, use Cloud Compare an the "Align Two Point Clouds" Workflow

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