[OSM-dev] Garmin GPX madness

Andy Robinson (blackadder-lists) ajrlists at googlemail.com
Mon Sep 22 17:35:24 BST 2008


Oliver Eichler wrote:
>Sent: 21 September 2008 6:26 PM
>To: dev at openstreetmap.org
>Subject: Re: [OSM-dev] Garmin GPX madness
>
>>
>> OK, I understand now. But I still don't know what I'd do with the
>> information if I had it. The satellites are where they are when I'm out
>> surveying, there's nothing I can do about the positional accuracy. It's
>> usually pretty obvious when a track has gone wildly off (usually due to
>> woodland cover or urban canyons), and I'm still generally going to use
>> the track otherwise.
>
>If you have all information and points recorded with a fixed sample rate
>you
>can apply algorithms to  minimize the error. But with the shit recorded by
>Garmin you can't do anything. It's spoiled data you have to take as is.
>Most
>users try to overcome this shortcoming by simply moving the points to the
>location they think it should be. But that is spoofing data the worst way.

Having spent time using both standard NMEA out devices and Garmin devices I
have a number of observations. These observations tell me that nothing that
any of the devices produces is shit. You might shit, I might shit but GPS
receivers really aren't clever enough to shit.

I find that devices that stream NMEA sentences which include HDOP data (I'm
ignoring VDOP on the basis that OSM has never sought to use GPS for height
information) require post processing before upload to OSM because they
regularly contain data that is wholly misleading, either because the
horizontal accuracy was very poor (high HDOP) or because it contains bad
data (partial sentences etc). Arguably out of the box a basic device only
outputting NMEA data is not a great OSM tool because of these limiters.
Agreed, if you spend the time to analyse/filter the raw data then you get a
more reliable result than with no analysis.

A Garmin device is not designed to be something that you analyse. The
analysis is done by the device itself before data is presented to the user
(on screen or as data output). Obviously if you want to make the analysis
decisions yourself then you will go and buy a device that punts out NMEA and
do it yourself, otherwise you pick the best tool for the job you are doing
and go mapping. So, a Garmin device may not be doing the analysis in the way
you would prefer, but at least its doing it.

Having tried both approaches over the last three years of my activity with
OSM I can safely say that both approaches work. Both have their limitations
(NMEA needs filtering, Garmin data will include some misleading data under
trees etc.) but overall neither is perfect but both are more than good
enough for OSM. The newer high sensitivity receivers (in all devices) have
made the question of accuracy rather a moot point. Regardless of device we
can get streets and objects placed where we need them on the map, even under
tree cover and even in urban canyons. Surely that's all we are concerned
about here? OSM is not meant to be the product that sets objects at 2.5cm
accuracy. 10m is good enough to place objects relative to each other and it
is relative placement surely that is important in our map, not positional
accuracy.

Cheers

Andy

>
>Oliver
>
>
>
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