Comment by amluto | Hacker News Reader

Comment by amluto | Hacker News Reader

I’m slowly making a map of my back yard in QGIS. One annoyance: I live in a place where tectonic movement over my expected lifespan is significant on the scale of my map. And so are the supposed inaccuracies in various coordinate systems. (I want to record where things are so I can find them again without digging big holes!)

As far as I can tell, QGIS has no particular understanding of either a coordinate relative to the (moving!) crust or of a coordinate in space-time that can be projected to space at future or past times. Surely this should be a thing!

I found HTDP, a web tool that can shift coordinates forward and back in time:

https://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml

And I found this discussion:

https://www.gpsworld.com/the-effects-of-tectonic-plate-movem...

But I haven’t found anything easy to use.

(I have an RTK-capable GPS and NTRIP data via UNAVCO from a nearby CORS station. I was hoping that storing position relative to such-and-such CORS station as it was on such—and-such date would be straightforwardly doable in QGIS.)

Wow that’s a pretty interesting (and scary) challenge.

I know that the coordinate systems get revised every few years. In Australia we used to use AGD83 then it was revised to GDA94 and more recently to GDA2020 — and this was to account for tectonic shifts. And there’s calculations for transforming from one to the other.

That’s the only clues I have on that side of things and I’ve forgotten most of what I’ve known.

Also there’s a concept called “rubber sheeting” for transforming points where you know the amount of error at a bunch of points and you want to transform all the other points by interpolating how much error there would be at those points. The might be useful too, if you’re implementing a solution yourself.

Specifically https://plugins.qgis.org/plugins/VectorBender/

Keep in mind too that GDA2020 has an inbuilt calculation for drift and will keep your coordinates up to date as time progresses.

I didn’t find anything in QGIS to give a layer a CRS and an associated date, so I’m not sure how coordinate systems like this are expected to work properly.

Generally speaking to set a CRS its: Right click on the vector layer > Set Layer CRS.

Alternatively: right click layer > save as, specify in properties.

There are geoprocessing tools to do the same.

Sometimes the input data may not be suitable for setting a CRS.

I am doing the same, Mainly I wanted to record where pipes and such are located but might as well record buildings, fences and trees while there. I was not worried about geologic movement but all the global coordinate systems felt very clunky when locally mapping at the centimeter level, I finally settled on having a well defined surveyed reference point, and using meters north and east off that.

Perhaps that would work for you. because while the plate is shifting with respect to the globe, everything on that plate will maintain the same relative position.

Wow, that makes three of us!

I set up my own NTRIP base station at a fixed point in the middle of my roof with a hefty bracket, did a PPP survey to determine its location once, and am considering everything relative to that. This spring when I'm back at the surveying, I'll likely do another PPP run and make sure it hasn't moved too much. If it has, I guess I'll have to figure out how to reconcile that. From what I've gathered, I don't think this is too far off from how real surveying works.

I didn't see much reason to use a different unit than degrees. Although while I haven't gotten super in depth learning QGIS, it feels like there's an impedance mismatch in that it seems to be a 2D program that treats degrees as linear units (and then applies a fudge factor to degrees longitude), rather than a native 3D program. So I'm doing all of my collection, point storage, and calculation with scripts outside of QGIS, and then only pushing the cooked results to QGIS for visualization.

I don’t have a good sense for how far one needs to be from one’s NTRIP base station, but I’m pretty happy with the UNAVCO data I’m getting. The nearest station is operated by Serious Professionals, and it won’t move if I mess around with my roof. :)

I've got a professional base station that's decently close, but still far enough to degrade the accuracy a bit (I forget the exact figure but you can find it if you search). When I used the professional base station to figure out the point and compared it to what PPP had found, it seemed to be within that amount of error.

I chose the part of the roof least likely to be disrupted. But if I do need to move it, then I'll have to figure out exactly how I want to map to points going forward.

Instead of a reference + north/east meters, how about two reference points, and then everything is referenced as polar coordinates from there, point A to point B being 0 degrees. My concern would be if the direction of north/east changed over time from the plate movement.

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