3 ms·

This mentioned Google hasn’t updated to this change, do we know when they will?

saalweachter 7y ago
The trickiness is going to be two-fold. The trickiest part is going to be that some previous GPS coordinates will have been "pinned" to the 1994 grid, and others will have been taken live over that time span. See, surveyors have always known that you can't actually just write down the GPS coordinates of a boundary or intersection or bridge, because of this exact problem. So surveyors do the math (well, most likely their specialized surveying GPS equipment does the math) and write down the coordinates relative to the baseline for the continental plate in question. If you have coordinates you know were LAT, LNG in the 1994 baseline, it's easy to apply the transform and get the LAT', LNG' in the new 2020 baseline. But what happens if your coordinates weren't provided by a surveyor carefully surveying a location in the appropriate baseline? What if your coordinates were measured by a jerk walking to a place last year with a GPS and writing down whatever was displayed? Those coordinates don't need to be transformed, they were already basically correct for the 2020 baseline. If you add the correction to those numbers you're introducing a new error. And to make matters worse, people have been entering non-calibrated numbers into your data set for 25 years -- some numbers will have 5 years of drift, some 15 years. If you know when an uncalibrated measurement was made, you can kind-of interpolate the drift, but without that you're just SoL. To make matters worse, a company like Google is using data from a wide variety of sources, some they gathered themselves, some they get from others. With your own data, you might at least know either whether it was calibrated to the previous baseline or at least when it was collected, but someone else's dataset might be messy in unknownable ways. Say you previously got the location of a park entrance from a government parks department, in the 1994 coordinates. Now you go through your aggregated dataset and apply the transformation to the coordinates and get the new number in 2020 coordinates (which thankfully will match what a consumer GPS actually says, now). Next month, when you update your data from the government parks department, it gives you a different coordinate for the park entrance, which is neither the number you had previously for the entrance in 1994 coordinates nor the number you computed in 2020 coordinates. Now, do you assume that new number is in the new coordinate system, or might it still have been measured in the old coordinate system, either having been measured prior to the changeover or just on old equipment that was not updated? Ideally your interchange format would include the time a GPS measurement was taken as well as any baseline it was calibrated to, but do you want to guess how often interchange formats are "ideal"? So, long story short, it's going to be a long time before most of the old coordinates are flushed from the system, and you'll probably occasionally see a "wrong" coordinate that can be traced to this sort of thing every now and again for the rest of your life.
- ferzul 7y ago
  the moral to this story is don't dilute your data. if your data is delivered in gda94, don't convert it to gps coords so all your data is in the same format. instead, when you render it, render it from gda94 into your own coordinate system. that way, you won't have a problem with interpreting what it means. there's no such thing as 145°E 40°S, but there is “145°E 45°S gps 2016-05-07” and that can forever be handled.
  - netsharc 7y ago
    In essence, we need 4 dimensional coordinates. Google Maps' Timeline feature has a bug where changes to place names propagate to the past. A bakery where I used to get lunch has changed to a burger place, so if I look back to where I had lunch on that day 2 years ago, it would mislead me into thinking I got a burger...
- Thorentis 7y ago
  I hadn't thought of this, thanks for explaining it so thoroughly. Could you explain though, how you record LAT, LNG coords relative to a continental plate baseline? I thought to solve this problem you simply recorded the time that the measurement was taken, so that you can update the coords based on known plate position data at a given time (e.g. if you know a position was recorded somewhere in Aus in 2018, you just add 2cm x 7cm/yr = 14cm north east to the measurement to get the 2020 coords).
  - saalweachter 7y ago
    That's honestly getting to a point I only know vaguely about -- my real-world experience is just from chatting with surveyors when I needed to get a couple of parcels of land surveyed (once for a boundary survey, wherein I'm not sure this came up, and once for a FEMA/flood zone survey, where matching the site data to the FEMA data was much more important). Part of the answer, in the US, (and the expensive equipment -- one of the smaller surveyors I talked to just rented it on an as-needed basis) are the Continuously Operating Reference Stations, which is a network of ground-based stations of defined positions which are used to refine GPS coordinates relative to the National Spatial Reference System: https://www.ngs.noaa.gov/CORS/ https://www.ngs.noaa.gov/CORS/

The trickiness is going to be two-fold. The trickiest part is going to be that some previous GPS coordinates will have been "pinned" to the 1994 grid, and others will have been taken live over that time span. See, surveyors have always known that you can't actually just write down the GPS coordinates of a boundary or intersection or bridge, because of this exact problem. So surveyors do the math (well, most likely their specialized surveying GPS equipment does the math) and write down the coordinates relative to the baseline for the continental plate in question. If you have coordinates you know were LAT, LNG in the 1994 baseline, it's easy to apply the transform and get the LAT', LNG' in the new 2020 baseline. But what happens if your coordinates weren't provided by a surveyor carefully surveying a location in the appropriate baseline? What if your coordinates were measured by a jerk walking to a place last year with a GPS and writing down whatever was displayed? Those coordinates don't need to be transformed, they were already basically correct for the 2020 baseline. If you add the correction to those numbers you're introducing a new error. And to make matters worse, people have been entering non-calibrated numbers into your data set for 25 years -- some numbers will have 5 years of drift, some 15 years. If you know when an uncalibrated measurement was made, you can kind-of interpolate the drift, but without that you're just SoL. To make matters worse, a company like Google is using data from a wide variety of sources, some they gathered themselves, some they get from others. With your own data, you might at least know either whether it was calibrated to the previous baseline or at least when it was collected, but someone else's dataset might be messy in unknownable ways. Say you previously got the location of a park entrance from a government parks department, in the 1994 coordinates. Now you go through your aggregated dataset and apply the transformation to the coordinates and get the new number in 2020 coordinates (which thankfully will match what a consumer GPS actually says, now). Next month, when you update your data from the government parks department, it gives you a different coordinate for the park entrance, which is neither the number you had previously for the entrance in 1994 coordinates nor the number you computed in 2020 coordinates. Now, do you assume that new number is in the new coordinate system, or might it still have been measured in the old coordinate system, either having been measured prior to the changeover or just on old equipment that was not updated? Ideally your interchange format would include the time a GPS measurement was taken as well as any baseline it was calibrated to, but do you want to guess how often interchange formats are "ideal"? So, long story short, it's going to be a long time before most of the old coordinates are flushed from the system, and you'll probably occasionally see a "wrong" coordinate that can be traced to this sort of thing every now and again for the rest of your life.

ferzul 7y ago
the moral to this story is don't dilute your data. if your data is delivered in gda94, don't convert it to gps coords so all your data is in the same format. instead, when you render it, render it from gda94 into your own coordinate system. that way, you won't have a problem with interpreting what it means. there's no such thing as 145°E 40°S, but there is “145°E 45°S gps 2016-05-07” and that can forever be handled.
- netsharc 7y ago
  In essence, we need 4 dimensional coordinates. Google Maps' Timeline feature has a bug where changes to place names propagate to the past. A bakery where I used to get lunch has changed to a burger place, so if I look back to where I had lunch on that day 2 years ago, it would mislead me into thinking I got a burger...
Thorentis 7y ago
I hadn't thought of this, thanks for explaining it so thoroughly. Could you explain though, how you record LAT, LNG coords relative to a continental plate baseline? I thought to solve this problem you simply recorded the time that the measurement was taken, so that you can update the coords based on known plate position data at a given time (e.g. if you know a position was recorded somewhere in Aus in 2018, you just add 2cm x 7cm/yr = 14cm north east to the measurement to get the 2020 coords).
- saalweachter 7y ago
  That's honestly getting to a point I only know vaguely about -- my real-world experience is just from chatting with surveyors when I needed to get a couple of parcels of land surveyed (once for a boundary survey, wherein I'm not sure this came up, and once for a FEMA/flood zone survey, where matching the site data to the FEMA data was much more important). Part of the answer, in the US, (and the expensive equipment -- one of the smaller surveyors I talked to just rented it on an as-needed basis) are the Continuously Operating Reference Stations, which is a network of ground-based stations of defined positions which are used to refine GPS coordinates relative to the National Spatial Reference System: https://www.ngs.noaa.gov/CORS/ https://www.ngs.noaa.gov/CORS/