[OSM-talk] GPX trackpoint basic inaccuracy

[Joerg Ostertag (OSM Munich/Germany)]

On Friday 15 September 2006 18:05, Nick Hill wrote:
> I have been creating queries to determine statistics of redundancy in
> the GPX trackpoints.
>
> At one point, we have a stack of 11385 gpx points in exactly the same
> place.
>
> There are 1259115 in stacks of 6 or more,
> 2533037 in stacks of 3 or more
> 6497729 in stacks of more than one
>
> Quantising the world into 0.000003 degree squares (about 30cm, 1
> imperial foot lat), and counting those trackpoints where there are more
> than one in a square, yields a redundancy of:
> 7297894 out of a total number of tiles 2655121 with more than one point.

...

I think most traditional GPS devices are working with nmea internally. 
The NMEA representation of coordinates is structured as follow
 ddmm.mm
where dd is the lat/lon in degrees and mm.mm is the amount of minutes to be 
added to the coordinate. So we have a quantization of 0.0001666 degrees 
directly comming from the gps. So this is our basic inaccuracy we probably 
have on every GPS position taken.
Probably at some GPS units it's then converted to a float or doublefloat. If 
this error increase is not done inside the GPS unit itself (for example to 
store the coordinates) you still have the chance to do it in:
  - the downloading program
  - the gpx format (storing floats)
  - the converting program
  - THE osm-api
  - the java interpreter inside josm
  - <put in your favourite lat/lon handling>
  - 

-

Joerg