Colossal Forces – Changing Directions (Wellington) EarthCache

This EarthCache is situated on the narrow Rongotai isthmus (tombolo) between Kilbirnie / Lyall Bay and Miramar. The isthmus used to be an ancient harbour entrance, and Miramar was an island. The area is now the navigation hub of New Zealand.

Geology

Flanked by greywacke ridges, the low-lying areas originated as river valleys eroded along the shatter zones of north-south-trending faults. About 1½ million years ago, the Hutt River became diverted by a depression that had developed along the Wellington Fault (NW of here) and began to flow along the fault line, and then out to sea through a valley at the earthcache location.

Sea level fell worldwide during the glacial periods. This area was then some 11 km away from the coast. Much coarse rock material was brought down by both solifluxion and the Hutt River. The valley here soon became choked with gravel. Today some tens of metres of gravels are found in the Rongotai area.

Sea level rose in the interglacial periods. About 500,000 years ago, the sea began to flood into the depression through this area, forming the Wellington Harbour. In the 13th century, early Maori settlers in this area recorded that the harbour had two entrances: Te Au-a-Tane (the present Pencarrow entrance) and Te-Awa-a-Taia (through Evans and Lyall bays), separated by an island Motu-Kairangi (the present Miramar Peninsula).

Since then, Te-Awa-a-Taia has gradually become filled with sand and other material transported along the coastline from the rapidly-eroding hills of the Cape Terawhiti-Sinclair Head region. In about 1460 an earthquake resulted in an uplift of some 4½ metres, and Miramar was joined to Kilbirnie by what is now the Rongotai isthmus, sealing off the old harbour entrance. When Captain Cook arrived off Port Nicholson in 1773, he found Miramar to be a peninsula. In 1855, another massive earthquake (magnitude 8.1–8.2 centred on the Wairarapa Fault, some 20 km to the east) was accompanied by an uplift of 2 metres and added yet more land to the area. Stretches of exposed rocky sea bed can be seen around the coastline in Moa Point and in the headland across Lyall Bay.

In fact, there have been a series of earlier co-seismic uplifts in this area. Eight Holocene palaeo-shorelines are recognised in Miramar and Rongotai, indicating a total uplift of about 10 metres over the past 7,000 years.

The 1855 earthquake generated a seismic sea wave (tsunami) with run-ups of 10 metres at Te Kopi in eastern Palliser Bay. The wave was 3-6 metres high in Wellington Harbour and swept in over much of the Kilbirnie-Rongotai area. Rongotai and Miramar were reportedly covered many times in water rushing in from Lyall Bay and from Evans Bay, to about one metre depth.

The geological ebb and flow of the sea and earth movements in this area are mirrored by its modern day function as NZ’s main navigational hub – Wellington Airport.

Wellington Airport

An airstrip had existed at Rongotai since the 1920s, but was closed down in 1947 for safety reasons as the grass surface often became unusable during winter. Wellington's gateway moved to Paraparaumu Airport (some 50 km to the north), which in 1949 became NZ’s busiest airport.

Modern airport rebuilding at Rongotai began in the 1950s, with a large hill at the northern end bulldozed into Evans Bay and a breakwater constructed at the southern end and on the Lyall Bay side to accommodate an extended runway. In 1959 Wellington’s new airport was opened. Today both ends of the runway at Evans and Lyall bays stand on reclaimed land.

Runway orientation

Fixed-wing aircraft takeoff and land best into the wind to minimise roll and reduce the ground speed needed to attain flying speed. Wellington Airport’s single runway was therefore constructed to align with the prevailing wind in a north-south orientation.

Runways are generally designated by the whole number nearest one-tenth of the azimuth (degrees divided by 10 rounded) of the runway centreline, measured clockwise from magnetic north. For example, Runway Three Four (34) aligns at roughly 340° magnetic; Runway One Six (16) is the designation for one with a magnetic SSE (157½°) heading; and Runway One Five (15) for 154°. Each digit is pronounced separately for clarity in radio communications.

As each runway is used in both directions, it has two numbers that are 18 (180°) apart. Thus, Runway 16 becomes Runway 34 when used in the opposite direction.

However, magnetic bearings drift over time (see below). Depending on the location and how much drift takes place, the runway designation may need to change. The same bearing drift will affect some runways more than others. For example, if the magnetic heading of a Runway 28 is 276°, even with an 8° increase, it will still be Runway 28. If however the original magnetic heading was 284°, then an increase of just 2° would mean the runway should become Runway 29. Such changes are not welcomed, as they require significant amendments to a wide range of navigational and descriptive documents. Because the drift is quite slow, runway designation changes are thankfully uncommon.

If magnetic bearings are problematical, why aren’t true north bearings used instead? Magnetic headings are generally used for air navigation since most common aircraft cockpit instruments are designed to use a compass or similar magnetic device.

Okay, but what are all these different “norths” anyway?

The three norths

Three north references are commonly in use:

True (geographic) north (TN) is the direction of a meridian of longitude that converges on the Geographic North Pole, which is defined by the International Terrestrial Reference System and is located approximately at the Earth's rotational axis. The axis wobbles slightly by a few metres, however.

Magnetic north (MN) is the direction indicated by a magnetic compass. However, MN is not fixed and moves slowly at a variable rate. In NZ, it is currently east of TN.

Grid north (GN) is the direction along the vertical grid lines of a map projection. For the New Zealand Map Grid (NZMG) and the New Zealand Transverse Mercator, these lines are parallel to E 173°, the central meridian of the projections.

The horizontal angular difference between TN and MN is called magnetic declination or variation. The angle between GN and MN is called grid magnetic angle. The one between GN and TN is called grid convergence. You apply these angles to convert between true, magnetic and grid bearings.

Magnetic declination

Declination arises from the fact that the Magnetic North Pole is located in a different area from the Geographic North Pole. So generally, the higher the latitude, the larger is the declination. Originating in the deep outer core, the Earth's magnetic field has a complex shape. Declination is also affected by solar factors and local anomalies originating in the upper mantle, crust or surface (e.g. iron deposits and magnetite), geological features (e.g. faults and lava beds) and topographical features (e.g. ridges and mountains). Spatial variation from place to place therefore reflects geomagnetic flows irregularities as well as local anomalies.

A further complication is deviation, caused by metal objects influencing the magnetic compass, resulting in the needle pointing in the wrong direction. These objects may include ships, aircraft, power lines, pipes, rails, buildings, and personal items such as crampons, steel watch or even your belt buckle.

Secular changes to geomagnetic flows result in slow changes to the field strength and directions. As the magnetic poles slowly drift on the Earth's surface, declination in a given area changes slowly over time. This can be important if using magnetic bearings from old charts or directions to metes in old deeds for locating places with any precision, as well as for airport runway designations described above. Declination changes can be quite large. In Wellington, for example, the declination was changing by more than ½° every five years according to NZMS 260 Topographical Map R27 (1983 Edition). On the other hand, the change is almost zero in some places in the world.

How people have managed these changes

The first known determination of declination was made in China in about 720 AD during the Tang Dynasty. Around 200 BC, the Chinese had discovered the tendency of a magnet to align itself in a north-south direction, and invented the first compass. A few centuries later, they discovered that MN did not necessarily coincide with TN, and invented the feng jin (slit needle) to read 7½° before the magnetic reading. This was probably the approximate declination value in China about 1,300 years ago. The technology evolved with the invention of the San He Luo Pan, an astronomical compass with one magnetic needle (zheng jin - proper needle) and two rings: the inner ring (di pan - earth plate) and the outer ring (tian pan – heaven plate). Marked with the 24 ancient Chinese cardinal directions, the rings were offset at half a cardinal position (7½°). Thus, the heaven plate reads the direction with reference to TN and the earth plate reads with reference to MN. Later, people found that the fixed 7½° did not agree with the changing declination and used the sun's shadow at noon to read the astronomical direction. Instead of using the tian pan, a sundial was used in addition to the luo pan.

In Europe, the concept of declination was known in the early 1400s, but its first precise measurement was not made until 1510, when Georg Hartmann determined the declination in Rome. The importance of declination for navigation was obvious. Mariners devised methods to determine its values and began compiling these values from locations around the world. In 1700 Edmund Halley came up with the idea of showing declination as contour lines on a map and used this novel concept to produce the first declination chart of the Atlantic Ocean. Declination charts have been produced regularly ever since.

The following charts show the declination in different places and at different times.

Earth magnetic field declination: 1590 to 1990

Declination – Australia and New Zealand: (date unknown)

Declination – New Zealand 1983

Declination – New Zealand 2005

Grid magnetic angle – North Island 2005

Note that NZ topo maps (and the last chart above) don’t show the true declination values, but give the grid magnetic angle instead. For example, NZMS 260 Topographical Map R27 (Wellington) Edition 2 states that “Magnetic North on this map is 22½° (400 mils) east of Grid North during 1983, increasing at the rate of approx. ½° (9 mils) over 5 years.” To obtain the true declination you need to adjust the grid magnetic angle by the convergence.

Note also that the rate of change itself changes with time. Using historical rates to update the declination on an old map is likely to result in error.

While it is not possible to predict magnetic values exactly, scientists can describe the Earth’s current magnetic field from observatory and satellite measurements, and how it has changed from previously. Updated with the latest information, large-scale geomagnetic models are calculated internationally on a 5-year basis. These models include an estimate of how the field is changing with time, but there may be changes in merely 5 or 10 years that can't be foreseen. Furthermore, the models are global in scope and do not attempt to cater for local anomalies mentioned above. Thus predictions can be somewhat unreliable.

Air navigation

As mentioned above, magnetic headings are generally used for air navigation since the most common aircraft cockpit instruments are based on magnetic devices. However, aviation maps, charts and databases are generally based on TN. A pilot in most small planes will plot a course using TN on a map, and then convert to magnetic bearings for in-plane use.

Changes in values require declination charts and databases to be updated at least twice a year. Ground-based radio-navigation aids are also checked and updated to keep them aligned with MN to allow pilots to use their compasses for accurate and reliable in-plane navigation.

GPS systems used for air navigation can use MN or TN. The receiver natively reads in TN, but can elegantly calculate MN readings based on its current true location and updated declination data tables. This function is present in some handheld GPS devices, but their data tables have been preset and cannot be easily updated. Instead a user north reference mode is provided where the value of the magnetic variation can be manually entered.

EarthCache activities

The given waypoints along Moa Point Road are aligned in parallel to the runway. Be careful when you approach them and stay well away from the roadway as far as possible. WP-S in particular could be quite close to traffic.

1. Go to WP-S (S 41° 20.147 E 174° 48.308).
2. Use a compass to take the magnetic bearing from WP-S to WP-N (S 41° 19.749 E 174° 48.335). You should aim your heading along the centre line of Moa Point Road. Call this M°.
3. Use your GPS to read the true bearing to WP-N. Call this T°.
4. Calculate the current declination: D° = T°-M°.
5. Go to WP-N, and measure the reverse bearings to WP-S and get a new set of numbers M2°, T2° and D2°.
6. Verify that M and M2 differ by 180; T and T2 differ by 180; and D = D2.
7. Calculate the magnetic heading of Runway 34 at the time when the airport was opened in 1959, given that the then declination in Wellington was about +19½°. Call this M1959°.
8. Express WP-S and WP-N in NZMG coordinates, and calculate the grid bearing from WP-S to WP-N. Call this G°. (Alternatively, use a protractor to measure G° on NZ Topo Map Sheet R27.)
9. Use the “NZ Revised 2008” formula to calculate the grid convergence = G°-T°.
10. Calculate the current grid magnetic angle: GMA° = D° + grid convergence.
11. Verify GMA° = G°-M°.
12. Compare your current GMA° with the one given on Topo Map R27 (Edition 2). What is your measured rate of change per year since 1983?
13. The rate given on the 1983 Topo Map was “+9 mils over 5 years” or +0.10° per year. Is your answer to step 12 similar?
14. If the rate of change given on the 1983 Topo Map (step 13) could be projected to the future, about which year will the Wellington Airport runway numbers need to change?
15. If you project the rate of change you measured above (step 12), when will the runway number change have to occur?
16. Identify a stretch of exposed rocky sea bed around the coastline in Moa Point or in the headland across Lyall Bay, take a photo of yourself (with GPS) pointing to it.

Formulae

Declination (D) = True bearing (T) - Magnetic bearing (M)
Magnetic bearing (M) = Compass bearing (C) + Magnetic deviation (d)
D = T - C - d
Grid magnetic angle (GMA) = Grid bearing (G) – Magnetic bearing (M)

D > 0 when MN is east of TN (i.e. M < T); similarly, GMA > 0 when MN is east of GN (i.e. M < G): the case in NZ.

For NZ prior to 29 August 2008 and for Northern Hemisphere (“conventional”):
Grid convergence = True bearing (T) - Grid bearing (G)
Grid convergence = T – (GMA + M) = (T – M) – GMA = D – GMA
Declination (D) = GMA + Grid convergence

For NZ since 29 August 2008 (“NZ revised 2008”):
Grid convergence = Grid bearing (G) – True bearing (T)
Grid convergence = (GMA + M) – T = GMA – (T – M) = GMA – D
Declination (D) = GMA – Grid convergence

In NZ topo map projections, GN differs from TN everywhere except along E 173°, the central meridian. Applying the “conventional” formula prior to 2008, map locations west of E 173° (T > G) had positive (easterly) convergence; locations east of E 173° (T < G) had negative (westerly) convergence. This signage was opposite to that in the Northern Hemisphere, where locations east of the central meridian have positive convergence. This is because the convergence of meridians in NZ is towards the South Pole, rather than the North Pole. In 2008, Land Information New Zealand changed this sign convention (with Standard LINZS25002) so that convergence is now positive for locations east of the central meridian, aligning with the Northern Hemisphere in this respect. But on the other hand, the algebraic formula has to be flipped around as indicated above.

Logging requirements

To claim your find, complete the above EarthCache activities and email us your answers to the following:

1. M°
2. T°
3. D°
4. M1959°
5. G°
6. Grid convergence°
7. GMA°
8. Your measured rate of GMA° change per year since 1983
9. About which year will the Wellington Airport runway numbers need to change, based on projecting the GMA° rate of change given on the 1983 Topo Map?
10. Which year will that be, based on projecting the rate you measured?

Please wait for our confirmation before logging your find. With your log, post a photo of yourself (and the GPS) pointing to the stretch of exposed rocky sea bed you have identified around the coastline in Moa Point or in the headland across Lyall Bay.