A note on distances

 This is as much a note to myself as anything else. This has been nagging at me for years so now that I got around to finding an answer, I shall record it here lest I need to look it up again.

For a long time, I was finding that the distance of a track recorded on my GPS (a Garmin GPSMap 64s) would come out significantly lower when downloaded onto my computer and opened in Memory-Map. My theory was that this was something to do with how Memory-Map opens the GPX file, maybe losing detail that the GOS unit recorded. As an example, on my recent Tutuwai tramp, my GPS showed 16.2 km when I arrived at the hut. Imported into Memory Map, the distance became 14.5km.

A track, in the form of a GPX file is essentially a list of recorded waypoints. The GPS records a point at certain intervals, stringing them together for a track. The GPX file can them be downloaded and imported into various bits of mapping software. The recording interval can be set on most GPS units - Garmin offer the options of manually specified time intervals (e.g. every 5 seconds), distance intervals (e.g. every 50 m), and an Auto mode with options to record more or less data. These option determine how many points will be in the GPX file. Most software will calculate the total distance by drawing straight lines between the points.

Eventually I determined that no information was being lost when I imported the GPX file, so the discrepancies I was getting must be due to something the GPS was doing. Today I stumbled upon what that was: the GPS is doing something along the lines of recording the same trip in two ways. The GPX file is recorded at the specified interval, and this is the distance that shows up when the GPX file is imported. The GPS is also recording the trip at 1 second intervals and this is the distance that shows up on the trip odometer and when reviewing the trip file on the GPS itself. More data points = more detail = longer distances. Fewer data points in the GPX = less detail = shorter distances.
Why the GPS unit operates like this I do not know. Whether one method is more objectively accurate is a mystery to me. That being said, 1s is very very frequent, to the point where it may be capturing a lot of positional jitter from the GPS, potentially over-estimating distance. And setting the GPS to record points at a lower frequency can under-estimate distance by smoothing out small meanderings etc.

A few forums I read seemed to like using the "Most often" option of the "Auto" track recording setting for Garmin units, so I'll suppose I'll do that from now on. And if nothing else, maybe someone else will stumble upon this and answer there own questions about GPX anomalies.

https://www.walklakes.co.uk/forum/viewtopic.php?f=16&t=571

Tutuwai Hut via Smith Creek

Not for the first time, I found myself cursing my decision to not wear full length gaiters. On this occasion, it was due largely to biblical plague-level amount of hook grass I had to wade through on the way to Tutuwai Hut.

 Some time earlier, I was strapping on my pack and heading off from the Kiwi Ranch Road carpark. The track initially passes though forest adjacent to a YMCA camp, zig-zagging upwards into lower-growing scrub. After about 15 minutes, a seemingly needless sign points the way onward, or back to the carpark. Checking the map later, I presume this is where the track from the old Marchant Rd carpark would've joined.

The track dips in and out of patches of forest on the way to Puffer Saddle. A further 15 minutes from the first sign is another - this one serving an actual purpose, pointing the way to Smith Creek or Alpha Hut. 

Taking the right path, the track continues toward Puffer Saddle. Many parts of the track surface were a horrible slippery clay stuff. It's another 15 minutes (for a total of 45 min/2.4km) to the saddle.

Shortly after the saddle, the track drops steeply down toward Smith Creek. At the bottom, there's a great spot for a break next to pleasant stream. From here, the track is mostly well graded and easy-going, but with plenty of side streams to cross. Closer to Smith Creek, a couple of areas affect by slips are traversed. Some of the slips are pretty major but the track only has a few places that are in any way hairy. About 2 hours/6.75km after starting, Smith Creek shelter is reached.

After crossing what would turn out to be the last bit of slip-damaged track, I encountered a sign warning that the track was closed and indicating a detour. The sign seemed to be intended for people heading the other direction, and I hadn't seen another at the southern end of the slip zone. Perhaps it got landslid-ed away. Regardless, I couldn't see much of a detour, and the lower closed track seemed fine. The sign looked relatively old so I imagine the slips have settled somewhat.

Smith Creek shelter is a dingy, uninviting place so I opted to take my rest stop slightly further down the track. From the shelter, the track follows the Tauherenikau river up stream, passing gorse-invested flats and swampy patches. A quick 1.2km later, Marchant Stream is reached. Marchant is one of the side streams to watch out for during heavy and/or prolonged rain - it drains a significant portion of the eastern side of Marchant Ridge. A guide wire stretches between banks, to lend support to trampers during high flows. Today though, it just helped me keep my feet dry. A few minutes later, the lower Tauherenikau bridge is reached.

As I was crossing the sturdy suspension bridge, I saw a couple of guys crossing the river. Where they were going I don't know, I never saw them again. From here, the track is a bit rougher. Soon after the bridge, there's a brief rooty scramble onto a terrace above the river.

There's an absence of undergrowth here that makes it easy to see a ground trail where none exists, so keep an eye on the track markers. The track meanders on terraces of varying height above the river, occasionally dropping or climbing into or out of side streams or onto different levels of terrace. There's a lot of tree fall and far too much hook grass. Occasionally landslides of varying age are encountered, none seem particularly recent so any potentially sketchy bits of track are pretty well bedded in, although there are a few slightly more scrambly patches. Some kind soul has strung up a rope at one such scramble.

Closer to the hut, a few flats covered in manuka and/or kanuka scrub are crossed. By this point, the sun was glaring overhead. There's a lot of boggy patches and plenty of mud. More cut up fallen trees, more hook grass. At one point, an old swing bridge abutment is passed. 

Just before the hut is grassy flat strewn with boulders. Further along is a good camping spot with a picnic table. There were also some large sack thing presumably dropped off by the helicopter I heard heading down the valley and containing supplies (such as firewood) for the hut.
The map makes it seem like the hut is approached directly from the south, so I expected to find it shortly after the flat. Instead, at a sign post, the track swings to the right. After a section of boardwalk and some stairs, Tutuwai Hut is reached on the edge of a terrace above the camping flat.

Helicopter delivery

A sign of hope

It took me 5 hours to cover the 15.2 km to the hut. The hut is tidy, and has the luxury of two (two!) inside sinks. No one else was present, so it was peaceful evening reading and relaxing. Once again I found myself ruing my gear choices upon realising I had packed my power bank, but not my phones charging cable. Hopefully 24% charge would survive long enough for photos tomorrow.

I had a surprisingly good sleep and surprisingly few aches and/or pains in the morning. I left just after 9am, the sun not yet high enough to bring the temperature to uncomfortable levels.

The walk out brought no surprises, and I was back at my car in 5:24.

Hut above the grassy river flats.

Approaching the rope slope

Good rest spot at the bottom of puffer saddle.

Map, etc.

Distance: 14.5 km each way. Time: 5:01 in,  5:24 out.