(Not to scale or orientation)
There are a variety of methods that mariners use to fix their position in coastal waters. Most typically, these involve taking bearings to fixed objects using the ships compass. However for this to work well, the navigator needs to take into account the compass error, which depends on maganetic variation (aka declination) at the general locality and the boat's heading. If the compass error is unknown, then another method can be used where compass error is not required - fixing by Horizontal Angles. (They could, of course, get out their GPS and just take the reading - but that's too easy and would be cheating).
When fixing by Horizontal Angles, bearings are taken to three fixed objects and the differences between these bearings are calculated. These angles are then dialled up on a Station Pointer tool which is then placed on the chart so that each of the three fingers passes through the three known locations on the chart. The centre hole in the elbow of the Station Pointer gives a position fix on the chart.
This cache is based on the same idea, and is set in Dove Mayer Robinson Park. The coordinates given above are for the carpark. The cache itself lies where the horizontal angles between the pictured objects match up. To work this out:
- Find the objects given in the pictures and record their coordinates. (Click on the links below for enlarged pictures).
- If your GPS can not be set to a grid projection (such as NZ Map Grid, or NZ Transverse Mercator, or UTM), then convert the geographical coordinates to grid coordinates at this LINZ site.
- Plot the grid coordinates on graph paper
- Use a simple station pointer to find the coordinates of the cache.
- Read off the coordinates for the cache from the graph.
- Convert these back to geographical coordinates (if they can't be directly entered into your GPS using the appropriate setting).
A simple station pointer device can be made by using tracing paper, or lunchwrap paper, with the bearing lines drawn on it from the middle of the sheet. When the bearing lines are all lined up on the three coordinates, the cache location will be where the bearing lines meet at the centre.
Technically, there is a slight difference in bearing measurements when projected onto a grid, but these are small at this scale and can be ignored. If you enjoy this cache, then have a go at the Horizontal Angles cache - which is the same problem, but on a larger scale.
You can check your answers for this puzzle on Geochecker.com.