Fan Brycheiniog Waypoints

Here’s a few useful grid refs from the waypoints I marked on my Fan Brycheiniog walk. They include a few summits and the locations of a few aircraft wrecks. Since EGNOS is now active these grid refs are surprisingly accurate when reception is good. On open high ground these days I typically get an EPE of 1m-3m, usually 2m. Most averaged waypoints have an EPE (estimated position error) of between 1.5m and 2m. Comparisons with accurate grid refs show these estimates to be pretty reliable.

The Pre-Amble

For those who aren’t familiar with how I do it, here’s an explanation. You can skip this bit if you already know or don’t really care. But I’ll point it out here for future reference.

When I record a grid ref I typically mark two waypoints, first a normal waypoint, then an averaged one. I rarely average for very long, usually only for about a minute, although sometimes longer if it’s an important grid ref, when I may stop for a while to rest, have a snack or take some photos.

Because my averages are usually quite short there may seem little point in recording them, but there are two good reasons. The first is to cancel out any spurious errors which can happen for a brief moment when contact is acquired or lost on a particular satellite, or due to the momentum factor immediately after stopping. The second is that the averaged waypoint records the GPS altitude, whereas the normal waypoint records the altimeter altitude (I have a GPS with a built in altimeter). So I need to do mark both types of waypoint to record both altitude readings.

Now that EGNOS is finally working, the accuracy is typically so good when there are no obstructions that the averaged waypoint is rarely more than 1m different to the normal waypoint, so there’s little need to record an averaged waypoint, unless you want to record the GPS altitude. For the grid refs listed below, I’ll choose the averaged waypoint in preference to the normal waypoint.

GPS altitude is less accurate though, usually x1.5 the horizontal error, so an EPE of 2m would give an altitude EPE of 3m. As for the altimeter accuracy, this depends on the weather conditions and on the GPS auto-calibration. As a result, the altimeter can sometimes be more accurate than the GPS altitude (typically in calm conditions), sometimes less accurate (typically in windy conditions).

To start with, I’ll just mention the waypoints I recorded at my car at the beginning and end of the walk. This is not to demonstrate absolute accuracy, but relative accuracy. I’ve included the high definition grid ref in brackets (GPS Utility is capable of showing this level of precision, which many GPS units record, but don’t actually display).

CAR1 SN 84576 16460 (SN 84576.00 16460.07) 208.0m
CAR2 SN 84576 16459 (SN 84575.64 16459.15) 208.0m

Note that the easting is identical and the northing is only 1m different. The high precision figures show the difference to be less than 1m. Also the altitude is identical, even to 1 decimal place. This is quite remarkable after a difference of 7 hours! Of course, such pairs of waypoints aren’t always this accurate, but it’s impressive none the less. It does help to inspire confidence in the level of accuracy that EGNOS provides.

Note that I mark all waypoints in a normal standing position, or on top of trig points, so I have to subtract one metre for the correct altitude. In this case that would make it 207m. If you check the grid ref against the 25k scale map, you’ll find that it’s roughly 7/10ths of the way between the 200m and 210m contours, pretty much exactly where you’d expect 207m to be in fact.

Having said that, I don’t usually get this level of accuracy from the altitude. On most of my walks the altitude tends to start too high. (I always calibrate the altitude at a known height at home before leaving, thus allowing plenty of time for the auto-calibration to take effect before starting the walk.)

I’ll subtract the 1m before listing the altitudes below.

The Grid References

Allt Fach 463m - SN 84150 17867
GPS altitude 464m (+1m), altimeter altitude 466m (+3m)

Fan Hir 761m - 83092 20928
GPS 763m (+2m), altimeter 770m (+9m)

Fan Brycheiniog / Fan Hir Col (height uncertain) - SN 82889 21301
GPS 720m, altimeter 718m

Fan Brycheiniog 802m - SN 82534 21798
GPS 807m (+5m), altimeter 806m (+4m)

Fan Brycheiniog Shelter (height uncertain) - SN 82552 21771
GPS 804m, altimeter 805m

Fan Brycheiniog Trig Point

This is worthy of more detail. The non-anally-retentive can ignore this bit.

Since this is a trig point I have an accurate grid ref supplied by the Ordnance Survey themselves. I have this in two formats, OSGB and WGS84. The OSGB grid ref is SN 82533 21792 and is the correct map format. The WGS84-lat/long format is the internal format used by GPS units and is N51.882288,W003.708040.

You’ll notice that the grid ref I recorded is inaccurate by 6m compared to the OSGB grid ref. However, most of that inaccuracy is due to the simple (and inaccurate) datum transformation that takes place in the GPS itself. Most commercially available GPS/mapping software (such as OziExplorer, GPS Utility, Memory Map, Anquet etc.) uses a similarly simple transformation.

The accurate transformation requires a complex formula. There is a free program available from the Ordnance Survey website which will accomplish this, but for most purposes it isn’t necessary.

The important thing is that the WGS84 grid ref is accurate, which is what GPS units use internally, irrespective of what datum and format the unit displays. Inputting the WGS84 grid ref, then converting it with software, will give pretty much the same result as the GPS unit does. This provides a far more accurate comparison.

Converting the WGS84 grid ref with GPS Utility in high precision mode (the most accurate “simple” conversion) gives a grid ref of SN 82535.73 21797.39, which rounds to SN 82536 21798. Compare this to my averaged EGNOS reading:

SN 82536 21798 - GPS Utility Conversion
SN 82534 21798 - GPS with EGNOS, averaged over about 5 minutes, EPE 1.7m.

As you can see, my recorded waypoint is only 2m different, consistent with the estimated position error. Pretty damn good I’d say!

In fact, I recorded another GPS waypoint and also took two photographs of my GPS on top of the trig point showing different readings. Averaging the three gives:
SN 82535 21798 - only 1m different! Even better!

One of the three photos actually shows SN 82536 21798, the precise grid ref (simple transformation). I could have just shown you that photo, but I didn’t want to imply greater accuracy than actually exists. (Update: I’ve now added that photo to the top of this post.)

None of the two waypoints I recorded and two photos I took were more than 2m out, so whilst I’d love to boast of sub-1m accuracy, we’re not quite there yet. But we’re not far off! EGNOS still has farther to go! [:-)]

The Aircraft Wrecks

The vampire wreck is quite easy to find, not far from the head of the river Haffes, it can be seen from about 500m away. The main wreckage is collected in one spot, but there’s also a second spot in a stream a short distance to the north-east. I also found some extra bits in a stream a bit further south. I’m not sure if the latter are commonly known.

VAMPIRE1 SN 82653 20042 604m This is the main wreckage
VAMPIRE2 SN 82703 20095 613m
VAMPIRE3 SN 82660 19894 599m

Further SSE, and much more difficult to find without a grid ref, are the very sparse remains of a Wellington bomber. I’ve also marked the small memorial cairn, which is much easier to see from a distance, although the wreckage itself isn’t, even from the cairn.

WELLINGTON SN 83251 18382 534m
MEM-CAIRN SN 83261 18389 538m

The Permissive Path

Finally, the permissive path which I didn’t know about, which I only discovered as a result of doing this walk. It’s a much gentler ascent to, and descent from, the Fan Hir ridge than the more well known one from Tafarn-y-Garreg, which passes Ty Hendrey farm. I’ve marked the stile at the top and at the bottom. The path is well-marked between these two waypoints.

SN 84032 17254 Top Stile
SN 84265 16745 Bottom Stile

Posted on 2007 05 31