How accurate are the OLC AGL traces?

On Thursday, July 16, 2020 at 6:28:25 AM UTC+2, kinsell wrote:
On 7/15/20 7:34 PM, MNLou wrote:
Hi Nick -

When I want to analyze a flight from OLC, I download the IGC file and open it in See You.

I think the AGL numbers are See You are more accurate than from the OLC summary.

YMMV -

Lou


The barometric sensor in the logger is going to be assuming 29.92 inches
of mercury for atmospheric pressure, any deviation from that is going to
introduce an error on top of the instrument accuracy. So the AGL
calculations can be off quite a bit no matter what database is used for
ground elevation..

I would suspect OLC is using GPS altitude for AGL calculations, and probably using a fairly course terrain database which would explain the inaccuracies.

On Thursday, July 16, 2020 at 2:19:19 AM UTC-4, wrote:
On Thursday, July 16, 2020 at 6:28:25 AM UTC+2, kinsell wrote:
On 7/15/20 7:34 PM, MNLou wrote:
Hi Nick -

When I want to analyze a flight from OLC, I download the IGC file and open it in See You.

I think the AGL numbers are See You are more accurate than from the OLC summary.

YMMV -

Lou


The barometric sensor in the logger is going to be assuming 29.92 inches
of mercury for atmospheric pressure, any deviation from that is going to
introduce an error on top of the instrument accuracy. So the AGL
calculations can be off quite a bit no matter what database is used for
ground elevation..

I would suspect OLC is using GPS altitude for AGL calculations, and probably using a fairly course terrain database which would explain the inaccuracies.

Moreover, I've seen many examples on OLC when it is mistaken on the takeoff time and place, but still assumes the altitude at the "takeoff" is that of the stated airport. This happens if, e.g., the logger does not have a GPS fix at the time of the real takeoff, or is reset later. So OLC "adjusts" the logged altitudes for its display. This results in grossly underestimated altitudes for the early part of the flight. (It apparently interpolates towards the landing altitude later in the flight.)

On 7/16/20 12:19 AM, wrote:
On Thursday, July 16, 2020 at 6:28:25 AM UTC+2, kinsell wrote:
On 7/15/20 7:34 PM, MNLou wrote:
Hi Nick -

When I want to analyze a flight from OLC, I download the IGC file and open it in See You.

I think the AGL numbers are See You are more accurate than from the OLC summary.

YMMV -

Lou


The barometric sensor in the logger is going to be assuming 29.92 inches
of mercury for atmospheric pressure, any deviation from that is going to
introduce an error on top of the instrument accuracy. So the AGL
calculations can be off quite a bit no matter what database is used for
ground elevation..

I would suspect OLC is using GPS altitude for AGL calculations, and probably using a fairly course terrain database which would explain the inaccuracies.


The large majority of traces submitted to OLC are going to have pressure
data but yes they do accept ones with only GPS altitude. It's possible
they always use GPS altitude, but it's less accurate and tends to read
higher in my experience.

Whichever they use, resolution is not the same as accuracy. Subtracting
two large, inaccurate numbers from each other does not provide
meaningful results. And yes people sometimes get too close to the rocks.

At 14:25 16 July 2020, kinsell wrote:
On 7/16/20 12:19 AM, wrote:
On Thursday, July 16, 2020 at 6:28:25 AM UTC+2, kinsell wrote:
On 7/15/20 7:34 PM, MNLou wrote:
Hi Nick -

When I want to analyze a flight from OLC, I download the IGC file and
open it in See You.

I think the AGL numbers are See You are more accurate than from the
OLC
summary.

YMMV -

Lou


The barometric sensor in the logger is going to be assuming 29.92
inches
of mercury for atmospheric pressure, any deviation from that is going
to
introduce an error on top of the instrument accuracy. So the AGL
calculations can be off quite a bit no matter what database is used
for
ground elevation..

I would suspect OLC is using GPS altitude for AGL calculations, and
probably using a fairly course terrain database which would explain the
inaccuracies.


The large majority of traces submitted to OLC are going to have pressure
data but yes they do accept ones with only GPS altitude. It's possible
they always use GPS altitude, but it's less accurate and tends to read
higher in my experience.

Whichever they use, resolution is not the same as accuracy. Subtracting
two large, inaccurate numbers from each other does not provide
meaningful results. And yes people sometimes get too close to the rocks.

GPS Altitudes from Flight Recorders _should_ be referenced to the WGS84
ellipsoid, but this is not always the case. There is usually a difference
between the WGS84 ellipsoid and Mean Sea Level.

What is the OLC Terrain Database referenced to?

Over flat lands, pretty good. Over mountains, well, it is "interesting". I have seen friends traces showing negative AGL when he was polishing his way up the mountains. I wouldn't put too much faith in the AGL reported by OLC for mountain flights where the pilots are working close to the mountains.

My two cents worth,
Steve Leonard