Charting Shark Island

[khiemtran]

Back to Shark Island again, this time on a mapping expedition with the sailing club...




It was a bit windier on Sunday, but we took the ferry and had a smooth ride from Circular Quay.



First, the team of young explorers scouted the island for possible Mirror landing sites...



Then they began to investigate some of the mysterious structures on the island.



The amenities are sparse on the island, but the adventure rides are awesome!



After lunch, it's time to begin charting the island. First we need to take bearings from landmarks around the harbour...



Then plot them on our charts, after compensating for magnetic declination (+13 degrees in Sydney).



When the lines meet is called a "resection".



Three lines gives you a "cocked hat" (a triangle which indicates your approximate position). The circle is Liem's original guess at the location of the island.



The kids did really well on their first attempt...



Not a bad effort!



After the mapping exercise, time for more exploring...



The wind is quite strong now, and we watch the older kids out in their 420s.



Before finally catching the ferry home.

Comments

[amaebi]

That looks like brilliant recreation.

[khiemtran]

It was! Even more so because we sail past the island all the time, but very rarely get to visit.

[asakiyume]

What is magnetic declination? And how does one take bearings? (I'm starting, clearly, from a place of entire ignorance)

Also, that door into the hill looks like it's decorated with shells--is that the case? (And what was its original purpose?)

[khiemtran]

We think that little building might have been an old toilet block (there's a much bigger, newer block further up the hill).

Magnetic declination is the difference between magnetic north and true north. In Sydney, the magnetic declination is 13 degrees, so when the compass looks like it's pointing north, it's actually pointing 13 degrees to the east. Magnetic declination varies all over the world and also changes from time to time.

Taking a bearing is quite simple, but also hard. You basically point the compass at a landmark, then read off the bearing. Some compasses will tell you the bearing straight away, others will require you to turn a bezel until it is aligned with the compass needle. Then you need to adjust the bearing for magnetic declination, which basically means adding 13 degrees (or twisting the bezel 13 degrees to the left). Then you plot the bearing on the map, by drawing a line backwards from the landmark at that angle. Then you just repeat from different landmarks, until you have enough lines to get a fix.

[asakiyume]

This is making me want to try orienteering!

[khiemtran]

It's definitely something worth trying at least once. There's very rarely any need for navigating with a map and compass these days, but the supplemental skills you'll learn - how to read a map and judge distances and identify landmarks are really useful life skills.

I'm always surprised when I meet people who can't do these things and who don't even realize just how much a map can tell you, beyond just "go left or right".

[asakiyume]

Once, when I was in fourth grade and our class did a stint of camping, I got a try--I liked it. But it would be fun to do it again as an adult.

I agree with you about people and maps. I'd much rather have a map than a GPS: with a map, you have a sense of yourself in a context, which you can't get if you're merely instructed to drive five miles, then turn right, then drive three more, then turn left, etc.

[khiemtran]

One thing I have found though, is that's fun to have the GPS on while exploring in the car. We found a few interesting places on our last holidays that we wouldn't have otherwise visited, especially nature reserves and trails that were a little way off the main roads.

[asakiyume]

Ah, because the GPS marks them, whereas maps don't?

[khiemtran]

It's probably more that the GPS map is easily visible while you're driving and also has more interesting features on it. For example, on our last holidays, when we were driving around Port Phillip Bay, we noticed a nature reserve not far off the main road we were on that we thought looked interesting enough to warrant a closer look. Access was via a series of small roads, which we probably would never have thought of going down. But when we got there, it turned out to be a great discovery and it's one of the places we'll definitely be going back to.