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>The shape of the asteroid's surface is topography. That's independent of the coordinate system. I would have stated this first. You don't really need a equip
by beerandt 7y ago
>The shape of the asteroid's surface is topography. That's independent of the coordinate system.
I would have stated this first.
You don't really need a equipotential surface or even vertical datum, unless you're trying to model potential energy, uphill and downhill, the flow of a river, or the gravity well. All of which are unlikely to be known or easily measurable on an astroid.
And if all you want to do is look at the geometry of a surface, you don't need, or even want to use a horizontal datum and projection. It will just overcomplicate the math.
Lat long or a similar polar (or elipsiodal) coordinate system using orthometric height from the origin (or an arbitrary "zero" datum height) will be much easier. Especially in the case of a body without existing coordinate systems or projections that need to be "tied-in."
On the other hand, there are cases where using the GPS style coordinate system makes more sense (which is just an x,y,z or x,y,H version of lat, long, height. Yes, at it's heart, GPS uses Cartesian x,y,z). This is especially true if you're not concerned with defining an ellipsoid, prime meridian or equator. (Especially true for shapes that might not rotate, or fit well to an elipsiod or sphere.)
The thing to remember is that coordinate systems and projections mainly exist to translate between the many different ways people have been mapping and measuring the Earth for hundreds or thousands of years. Sometimes there are good reasons for these different methods (shape, area, directional constraints), and sometimes it's advanced tech/ systems (SI vs Imperial, Clarke elipsiod vs Hayford , or convenience/locality (relating Rose line observations with those made in Greenwich), or national security (USA not wanting nuclear icbm targeting stolen by USSR to be useful/meaningful/etc).
I say that to say this: If you're doing everything from scratch, there's no need to complicate things. Stick to the simplest math that works for the projects demands. There's nothing wrong with spherical polar or xyz.
- beerandt 7y agoPut another way: don't go looking for a nail just because you have a hammer.
- jofer 7y agoVery true! You don't need a datum unless you're measuring angles on the surface and elevations. My point was more that the standard mapping tools allow non-Earth bodies just fine. E.g. here's a UTM-like projection for Mars using a simple ellipsoid in proj4: "+proj=tmerc +lat_0=0 +lon_0=0 +k=0.9996 +x_0=0 +y_0=0 +a=3396190 +b=3376200 +units=m +no_defs" You can happily define a simple sphere of the right general size and have things work correctly. It's not overcomplicating the math to use existing libraries, it's avoiding rewriting a lot of fairly complex operations that are easy to get wrong. You can use gdal, proj, org, et al quite well on other planets. It's a hell of a lot easier than writing everything from scratch. They're set up to do things like this.
- beerandt 7y agoYes, I didn't mean to imply that the software wouldn't be useful. (Or for anyone to attempt to do all the math on their own.) Most people just don't realize you can define a generic coordinate system, and depending on the software, leave the projections/ datums blank (or set a generic/custom one), and still get good use out of many of the tools. (The danger is some tools that should only be used with projections might run error free without having one defined and return a meaningless answer. Although admittedly I haven't kept up with the different capabilities for the last few years. What method/format would even be used to generate points for an astroid? Radar? Lidar? Photogrammetry? From a "where do you start" perspective, knowing that would probably narrow down practical approaches...