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I’m surprised that there isn’t GPS coordinate system which is just kilometers. Instead of 360 degrees uses 40,000 km. The real calculation would use the real di

I’m surprised that there isn’t GPS coordinate system which is just kilometers. Instead of 360 degrees uses 40,000 km. The real calculation would use the real distances but the approximation is close enough. This means don’t have to do any conversion to distances, at least for metric folks. One problem with degrees is that hard to convert to useful distances. This tricks help a lot, but it would be better to have no conversion.

dharmab 3y ago
The perfect sphere approximation of Earth is not at all close enough for GIS applications. Geoid data (the actual ellipsoid-ish shape with height variance) is an important factor in accurate GIS software.
jonsen 3y ago
> ...don’t have to do any conversion to distances... Are you advocating for the use of Manhattan distances for everything?
pezezin 3y ago
https://en.wikipedia.org/wiki/Earth-centered,_Earth-fixed_coordinate_system https://en.wikipedia.org/wiki/Earth-centered,_Earth-fixed_co... In a previous life I had to implement conversions between ECEF and WGS84. If you do it, make sure to use at least 64-bit floats, the Earth is so big that 32-bit arithmetic will introduce errors on the order of meters.
- sgarland 3y ago
  You’ve brought memories of dealing with NAD83 (for local maps) and WGS84 (for everything else) crashing back. The error between those is tiny at the distances I was dealing with, but it was enough that it bothered me. “I’ll just automate this mapping task in Python, how bad could it be?” - A very-new-to-everything me, years ago.
bmacho 3y ago
Do you have an example in mind? A problem that you may encounter, how do you calculate now, and how could you calculate with a GPS system that is just kilometers?
dagw 3y ago
There is a 'metric' coordinate system, it's called EPSG:3857 and uses meters as its units. Although it is not valid close to the poles and can give error of up to 0.6% so it's not useful for when you need high accuracy or to cover the entire globe. Most places in world also have local coordinate systems that reproject smaller geographic areas to cartesian coordinates with meter units for making easier to work in those areas.
jabl 3y ago
The main difficulty is that for ease of calculation you'd like a cartesian grid, which maps poorly to the spheroid shape of the earth. One solution is UTM https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_... which divides the earth into 60 zones, and then inside each zone you have a cartesian grid with meters as the unit. So a UTM coordinate consists of the zone designator, and then distances in meters from the equator and the zone's central meridian. There also MGRS https://en.wikipedia.org/wiki/Military_Grid_Reference_System https://en.wikipedia.org/wiki/Military_Grid_Reference_System , the NATO military variant of UTM.
perilunar 3y ago
Isn't that what gradians do? 360° = 400 gon, so 1 gon = 100 km of arc length https://en.wikipedia.org/wiki/Gradian#Relation_to_the_metre https://en.wikipedia.org/wiki/Gradian#Relation_to_the_metre

dharmab 3y ago

The perfect sphere approximation of Earth is not at all close enough for GIS applications. Geoid data (the actual ellipsoid-ish shape with height variance) is an important factor in accurate GIS software.

jonsen 3y ago

> ...don’t have to do any conversion to distances... Are you advocating for the use of Manhattan distances for everything?

pezezin 3y ago

https://en.wikipedia.org/wiki/Earth-centered,_Earth-fixed_coordinate_system https://en.wikipedia.org/wiki/Earth-centered,_Earth-fixed_co... In a previous life I had to implement conversions between ECEF and WGS84. If you do it, make sure to use at least 64-bit floats, the Earth is so big that 32-bit arithmetic will introduce errors on the order of meters.

sgarland 3y ago
You’ve brought memories of dealing with NAD83 (for local maps) and WGS84 (for everything else) crashing back. The error between those is tiny at the distances I was dealing with, but it was enough that it bothered me. “I’ll just automate this mapping task in Python, how bad could it be?” - A very-new-to-everything me, years ago.

bmacho 3y ago

Do you have an example in mind? A problem that you may encounter, how do you calculate now, and how could you calculate with a GPS system that is just kilometers?

dagw 3y ago

There is a 'metric' coordinate system, it's called EPSG:3857 and uses meters as its units. Although it is not valid close to the poles and can give error of up to 0.6% so it's not useful for when you need high accuracy or to cover the entire globe. Most places in world also have local coordinate systems that reproject smaller geographic areas to cartesian coordinates with meter units for making easier to work in those areas.

jabl 3y ago

The main difficulty is that for ease of calculation you'd like a cartesian grid, which maps poorly to the spheroid shape of the earth. One solution is UTM https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_... which divides the earth into 60 zones, and then inside each zone you have a cartesian grid with meters as the unit. So a UTM coordinate consists of the zone designator, and then distances in meters from the equator and the zone's central meridian. There also MGRS https://en.wikipedia.org/wiki/Military_Grid_Reference_System https://en.wikipedia.org/wiki/Military_Grid_Reference_System , the NATO military variant of UTM.

perilunar 3y ago

Isn't that what gradians do? 360° = 400 gon, so 1 gon = 100 km of arc length https://en.wikipedia.org/wiki/Gradian#Relation_to_the_metre https://en.wikipedia.org/wiki/Gradian#Relation_to_the_metre