Why my Forerunner 245 distance measurement are really bad?

I can't comment on your distance difference, but GPS smart is not every second. My experience is that precision improve significant by changing to every second and that is what I am using. But hardly enough to explain the difference.

As I wrote GPS smart is turned off.. And I set to record each second. 

So every second forerunner should take a point

Not sure if this helps, but Garmin watches don't support 3D distance, except for "outdoor watches" like Fenix (and its cousins, 935/945):

support.garmin.com/.../

support.garmin.com/.../

www8.garmin.com/.../GUID-69434E34-3AD0-41FA-B83B-ECDB462B72DC.html

WillNorthYork Thank you
That makes sense, may be a barometer is needed to calculate 3D Distance direct on the watch

But

As Forerunner 245 loads data on Garmin site (Connect) there is an elevation correction available, what is it for??

I made a comparison between real elevation (DTM) and the one downloaded as gpx from connect. There aren't big differences..

Then I made just now a new comparison

With the GPX downloaded from connect (7.99Km) I imported the track (that is a 3D track) into Postgis (spatial database) with a metric projection (EPSG 32633).

Then I used two spatial funcions ST_Length2D(geom) that returns 2D length of a linestring vector feature and the ST_3DLength(geom) function that calculates 3D distance of a 3D vector linear feature

The result is

Garmin Connect 7.99Km

ST_3DLength 10.05 Km (similar -50m- to 10.1Km calculated with QGis - same EPSG see above)

ST_Length2D 9.84Km

So, between 7.99Km and 9.84Km (assumed that these are both 2D distances) there is almost 2 Km difference and 2Km difference on 7.99 is 25% error - awful!

And of course I set to record a point each second on my watch...

Another test last week

I went for a sail trip/contest, so my path is completely flat

Forerunner measured 13.37nm (24.8Km)

GIS metric conversion and length measurement is 29Km

Almost 4Km difference on a 29Km path (14% difference....)

That sounds very strange. I can understand that you are not amused.

Of course I don’t know what is wrong with your watch, maybe a hardware defect. I can only comment on the accuracy of my FR245. I have used it for quite some time now with a couple of firmwares and GPS versions. Activity is running from Garmin with GPS & Glonass (default), with Smart recording (default).

There is one track, I run often, eventually once a week. It is in on a road in a wood, so some interference from trees. Measured with Google Maps, it is 10,3 km long. My FR245 has given me distances between 10,28 and 10,38 km for this track.

So all under 1%, mostly even less then 0,5%.

Do you use a standard activity for recording or something from the move IQ store? I have discussed a topic here in the forums, where a third party activity was showing wrong values, while the Garmin activity was correct.

Hi

I use standard garmin activity. 

I wrote to garmin support and they told me to flash latest firmware (9.6) and do a full format

We will see what happens.. 

Thanks

Good luck.

Maybe it is a stupid question, but did you have a full lock on GPS when you started your activities? (FR 245 confirms that with „green bar“ on the top, searching usually takes 20 seconds up to a minute)

The only time I had such big differences in measured distance like you described, it was when I ran with a borrowed Polar device and did not wait for the GPS lock at start of the activity, because I was not familiar with it. Ran the 10,3 km and the watch said something like 7 or 8…

Yes I always wait for the GPS fix, green bar and beep.. 

Thnx