Tag Archives: Navigation

Course reconstruction charts

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Click on the thumbnail images below to switch between charts. From left to right the charts are:

  • Abel Tasman’s approach to New Zealand and progress north
  • Abel Tasman’s course into and out of golden Bay
  • Abel Tasman’s progress 19 December to 26 December 1642

Reconstructing Abel Tasman’s course

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At first glance, re-creating the course followed by the Heemskerck and the Zeehaen should be straightforward; Tasman’s journal has an entry for every day, and that entry includes a coordinate comprising a latitude and a longitude.

A typical daily location summary looks like this;

“At noon Latitude estimated 40° 13′, Longitude 192° 7′; course kept north-north-west, sailed 20 miles.”

The longitude given appears unlikely, but is actually just a different way of representing longitude. We are familiar with longitude coordinates in the range 180° to -180°, which we measure from the Meridian at Greenwich. Tasman however measured his longitude east from the peak of Tenerife Island, in the range 0 to 360.

Teide, the peak on Tenerife lies 16° 38′ to the west of Greenwich, so it is easy enough to adjust Tasman’s reported longitudes to derive coordinates that we can use in modern mapping systems. However, when we look at the course this yields, we get a disappointing result. The course lies nowhere near to coast of New Zealand.

This is because Tasman had no means of measuring longitude. His east-west movement each day was estimated by ‘dead reckoning’, and all errors were cumulative.

The latitudes and longitudes recorded in Tasman’s journal provide a very poor representation of the route sailed. It is possible however to reconstruct his course by using other details recorded in his journal. Below is an example.

At noon on 14 December Tasman’s journal recorded that he was in the latitude 42° 10′ S (the sun that day was “observed”) and also that he was “2 miles” off the coast. (Tasman measured distance in Dutch miles. 1 Dutch mile is 7.4km).

We can reliably reconstruct this position using his reported latitude, and that distance (14.8km) offshore.

At noon on 14 December Tasman reported his longitude as Longitude 189° 3′, which is 172° 22′ E relative to the Greenwich meridian. His actual longitude that day (2 Dutch miles off the coast) was 171° 8′, which is 1° 14′ different to his true position.

Tasman’s ‘longitude’ is discussed more fully in the post Tasman’s Navigation, Part 1

While Tasman’s longitude was always estimated, the same is not true of his reported latitude.

As long as Tasman was able to see the midday sun, he could calculate his latitude remarkably well. How he did this is described in the post Tasman’s Navigation, Part 2

When Tasman could sight the noon sun’s altitude he provided us with half of his actual location… the latitude. In the course reconstruction, other details from the journal are used in conjunction with this, to derive his longitude.

Locations derived from component parts

Locations derived from component parts

The noon location on 14 December is an example of this. He was known to be at a certain latitude, and also at a known distance from the shore. From these the full location, 42° 10′ S, 171° 8′ E can be derived.

The course reconstruction is created from all the spatial information sources recorded in the journal: observed latitude, bearings to physical features, distances to physical features, direction sailed, distance sailed, times, and depths. Estimated latitude, and longitude are ignored in favour of these.

It is not a completely precise reconstruction, this is not possible from the information available, but it is accurate within known bounds:

Observed latitude

Testing with contemporary replicas of 17th century navigational instruments, shows that both the cross-staff and back-staff could reliably be used to measure the suns altitude (and thereby determine latitude) to within 2 arc minutes (1 arc minute being 1/60th of a degree). However, this was only achievable on days when the sun’s altitude could be measured.

When the noon sun was visible, the latitude was recorded in the journal as “observed”. When the sun was obscured, the latitude was reported as “estimated”. On these occasions the recorded latitude was estimated by dead reckoning.

Bearings to features

Course bearings and bearings to features were recorded to the closest compass point. Tasman used a 32 point compass, which means that all bearings given have a confidence of +/- 5.6°. The 32 point compass is described in the post Tasman’s Navigation, Part 1

Distances to features

The unit of length used throughout the journal is the Dutch mile. There were fifteen Dutch miles per degree of latitude (or per degree of longitude measured at the equator).

A Dutch mile has the contemporary equivalent of 7.4km (4.6 Imperial miles).

Distances to physical features were judged, not measured, and these can have a high variability. However, analysis of the distances on the occasions that they can be verified, indicates a margin of error of +/- 30%

Distance sailed

 A 30 minute sand glass and a 'traverse board' used to record course sailed and duration through a single watch

A 30 minute sand glass and a ‘traverse board’ used to record course sailed and duration through a single watch

Distance ‘sailed’ recorded in the journal was the sum of the day’s dead reckonings of each watch. The dead reckoning for each watch was derived from the sum of the duration and courses held during that watch.

There is some significant variability in these, but wayward distances can be removed by comparison of the start and end coordinates, and the reported distance sailed. Discrepancies exceeding 1 Standard Deviation were excluded from the reconstruction.

Distance ‘sailed’ was always rounded to the nearest whole Dutch mile and therefore has an additional variability of +/- half a Dutch mile.

Depth

Depths were recorded while at sea and at anchor. At anchor, depth was measured while they were stationary using a line with knots at fathom intervals. This was quite accurate, but took no account of the state of the tide. At sea, depth was measured on the move using a longer rope, with knots at 5 fathom intervals. This has a variability of at least +/- 5 fathoms.

Time

Tasman had no clock, and measured the passage of time in 30 minute increments using sand glasses. The day was divided into 6 watches, with each watch lasting four hours, or 8 glasses.

Although Tasman only rarely recorded time as “o’clock”, the time of day can often be derived from watch information in the journal.

The reference “in the middle of the afternoon” for example, means half way through the afternoon watch, or 14:00.

Midday, or noon, was when the sun was at its highest at his current location. At this time they re-started turning the sand glass. But the ‘noon’ time changed as they moved east or west, and cannot be directly compared to the ‘time zone’ based standard we keep today.

If Tasman had carried a clock, then as he travelled up the New Zealand coast, the time on his clock would typically show nearly 1½ hours earlier than on ours.

By combining these component parts, it is possible to deduce or approximate Tasman’s noon position each day and at many additional locations.

These are used to prepare a detailed course reconstruction, which is presented in a series of charts in the following post.

First Land

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Abel Tasman's actual vs estimated position on Dec 13th 1642.

Abel Tasman’s actual vs estimated position on Dec 13th 1642.

On December 13th 1642, Abel Tasman was nowhere near where he thought he was.

While Tasman’s Latitude was correct, his longitude estimation between Tasmania and New Zealand was wrong by over a degree.

In his journal entry for 14th December Tasman wrote something that exposed this error.

He described being so close to the coast that he could see the surf breaking, yet according to his longitude estimate he was still 150 km from the nearest land.

At noon on 14th of December Tasman also put this in his journal; “At noon Latitude observed 42° 10′, Longitude 189°3′; course kept east, sailed 12 myles. We were about 2 myles off the coast.” (a Dutch ‘myle’ is one fifteenth of a degree of latitude, or 7.4km).

This allows us to reconstruct exactly where he was at noon on Dec 13th.

Abel Tasman's actual vs estimated position on Dec 13th 1642.

Abel Tasman’s actual vs estimated position on Dec 13th 1642.

He was 2 myles off the coast, and had sailed directly east 12 myles since the previous day… and for that day, Tasman had been able to make a sighting of the midday sun; and he recorded an ‘observed’ latitude 42° 10’s.

At midday on December 13th 1642, Abel Tasman was in the latitude 42° 10’s, 104 km off the coast, and heading south east towards ‘large, high-lying land’. The land was ‘at about 15 myles distance’; approximately 110 km.

So what was the high land had had seen and was sailing towards?

This analysis shows which land was visible to Tasman from his location at noon.

Geographic analysis of the land visible to Abel Tasman from his noon position on Dec 13th.

Geographic analysis showing the land visible to Abel Tasman from his noon position on Dec 13th.

The peaks nearer the coast, Mt Camelback and Mt Grahem were visible to him, but at 561m and 828m respectively they were very low on his horizon and most likely lost in the surface haze.

Most of his skyline was formed by the line of peaks in the middle distance; Cairn Peak (1,859), Mt Reeves (1,783m), Mt O’Connor (1,815m) and Mt Bowen (1,985). Tasman wrote that he saw land about 15 myles away, and this line of peaks is 17 ½ myles from his noon position.

The distant skyline filled the gaps between these peaks and comprises mountains at the northern end of the Southern Alps, including Mt Murchison (2,400m) and Urquarts Peak (2,118m). These peaks form a slightly higher horizon, but are further away; nearly 20 ‘myles’.

Abel Tasman could have seen the biggest peaks in the Southern Alps; Mt Cook and Mt Tasman. They were well above his horizon, and even though Mt Cook (3754m) was 160 km away, it’s perfectly possible to see it from this distance in favourable conditions.

Mount Cook seen from  Fourteen Mile beach, 140km away

Mount Cook seen from Fourteen Mile beach, 140km away

From his actual position on 13th December he could have seen Mt Cook and Mt Tasman; the biggest peaks in the Southern Alps, but he could not have seen them to his south east. If he had seen them, he would have recorded seeing them to his south-south-west.

When Tasman saw a ‘large, high-lying land’, he was not looking at that snow capped peak that now bears his name, but at Mount O’Connor, Mount Reeves, and Cairn Peak.

The first 'high land' that Abel Tasman saw.

The first ‘high-lying land’ that Abel Tasman saw.

This, is the ‘high-lying land’ that was seen by Abel Tasman.

The main peak appearing south east of Abel Tasman on Dec 13th 1642 was Mt O’Connor, 1,815m.

Tasman’s navigation: Part 2

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LONGITUDE

latitude and  longitude

Latitude and longitude on a globe.

Longitude was the big issue for Tasman. He simply had no way of measuring how far east or west he was. There wouldn’t be a method to readily resolve longitude until Cook carried the experimental Harrison ‘K1’ instrument on his second Pacific voyage in 1772. The Harrison ‘K1’ was a clock.

The successful resolution of longitude hinges on being able to know time very accurately.

The Italian explorer Amerigo Vespucci made an insightful observation during a voyage to the America’s.

“…one night, the twenty-third of August 1499, there was a conjunction of the moon with Mars, which according to the almanac was to occur at midnight or a half hour before. I found that…at midnight Mars’s position was three and a half degrees to the east.”

Vespucci was watching a transit of the moon by mars, but it didn’t happen at the time he expected.

The earth makes a complete turn on its axis every 24 hours, and in this time the sun makes a complete revolution of the earth (to the terrestrial observer). It rises in the east, sets in the west, and then becomes invisible below the horizon until it appears again in the east. ‘Noon’ is when the sun reaches the highest point in the sky at a given location… noon is not simultaneous at all places on Earth.

The Prime Meridian at Greenwich

Standing with one foot in the east, and one in the west, at the Royal Observatory, Greenwich

Harrison K1 'one day' watch

The Harrison ‘K1’ watch carried by James Cook on his second Pacific voyage.

To a terrestrial observer, ‘noon’, when the sun is at its highest, occurs every 24 hours… precisely.

The sun moves through the sky, east to west, at 15° per hour. If I travel west then ‘noon’ occurs later, if I travel east it occurs earlier. The time that noon occurs is directly related to the distance I travel east or west.

‘Noon’, at a location 15° to the west of me, will occur exactly an hour later than at my location.

If you had an accurate clock, and set it to 12:00 ‘midday’, exactly noon at your point of departure, then you could work out your longitude anywhere in the world.

When you observe noon at your current location, and compare this to the time on the clock, then the time difference tells you your current longitude. If noon locally occurs 1 hour before noon at your departure point (which is shown by the time on your clock) then you are 15° to the east of it.

1 hour time difference between local noon time and origin noon time = 15° difference in longitude. If you know the local time at any place on the earth, then you can calculate your longitude based on the difference between local time and the time at your origin.

By the time Cook first sailed the pacific in 1769 an astronomical method had been developed for reckoning the time at Greenwich based on observations of the moon and stars. It was important to know the time at Greenwich as this was the location the British chose as the line of 0° longitude for their charts. Known as the ‘Lunar Distance’ method it involved measuring the angular distance between a star and the moon, the star’s elevation above the horizon, and the moon’s elevation above the horizon. The technique was quite accurate, but difficult. It took about four hours to perform the necessary calculations, but it yielded the time at Greenwich when the observations were made.

The difference between ‘local time’ and ‘Greenwich time’ allowed Cook to calculate his longitude, but incredibly, Cook’s ‘local time’ was still based on hour-glasses, corrected daily by his observations of the sun. On his first voyage Cook had no clock of any description.

Regardless of how tedious the process was, the ‘Lunar distance’ method produced the required result.

These days we still use Greenwich as the zero point for longitude and as the ‘normal’ point for time zones, but this convention was not universally adopted until 1884.

James Cook, using the ‘Lunar distance’ method, was able to calculate his longitude with such confidence that he no longer sailed ‘lines of latitude’… he could set a course that took him directly to his objective.

When Cook set out on his second Pacific voyage in 1772 he carried the experiment ‘K1’ watch. It was such an accurate timekeeper that he later wrote to the Admiralty… “Mr Kendall’s watch has exceeded the

expectations of its most zealous advocate…”. With this reliable timepiece Cook could determine his longitude both reliably and quickly. All he had to do was find the local noon time, by observing the sun’s height, and then read from the K1 clock how far advanced, or retarded this was from 12:00. One minute of difference in these times represented a quarter of a degree of longitude distance from Greenwich.

By 1772 the longitude problem was solved, but when Abel Tasman was sailing 140 years earlier, and he had to make do with far less reliable techniques.

DEAD RECKONING

Tasman had no instrument from which to directly calculate his longitude, yet in his daily journal he recorded both his latitude and his longitude… so how was this done?

What he did was estimate how far he thought he had traveled since the previous day, and in what direction. He measured his latitude at noon every day, and recorded that, and then he used his estimate of distance and direction traveled and added this variation in longitude to his previous days entry.

His estimate of longitude was cumulative, based on daily estimates of speed and direction since his departure from a known location. Thus, any deficiency in his estimate was compounded. Over the course of his 5,500 km journey across the Indian Ocean, his estimate of longitude was in error by 670 km.

He made his estimate based on two things; his speed, and the direction he was travelling. If he knew his speed, and the direction he had traveled, then he could calculate from that how far east or west he had moved since the previous day.

Measuring a vessels speed

The equipment used to measure a vessels speed

Leeway diagram

‘Leeway’ is the downwind drift of the course caused by the pressure of the wind.

Duyfken hourglass

An hourglass on the replica of the Duyfken

This technique for estimating your position without direct measurements is called ‘dead reckoning’. However, these estimates of both speed, and direction had their difficulties.

He measured his speed with a spool of thin rope, knotted at set intervals and a floating baffle. It took three sailors to measure the speed. One held the rope spool, the second dropped the float into the water, the third turned a 30 second sand glass. The second person started counting knots as they passed through his hand, and at the end of the 30 seconds the count was recorded. The number of knots that had passed was the speed of the vessel.

The direction a sailing ship travels is not the same as the course it is sailing.

The issue here is ‘leeway’; that is, the sideways drift of a ship to leeward (downwind) of the steered course. If a ship is going sideways across the wind, then as the sails propel them forward, the pressure on

the sails pushes the ship sideways across the water in the downwind direction. Their ‘course’ is the direction they’re pointing, their ‘course made’ is the direction actually traveled. The difference is their leeway.

A ship with a deep and broad keel will have a smaller sideways slippage, or leeway, than a ship with a small narrow keel.

An experienced skipper will have a good understanding of his vessel’s leeway at any sailing angle, but this is only a ‘best guess’. A more reliable way to determine ‘course made’ is to look at the wake left by the vessel, and take its bearing from the ships compass.

Tasman’s estimate of how far he had traveled each day was most determined by how accurately his measurements represented the days sailing. On any given day they would change course several times, and for each of

these tacks; speed, direction and duration were noted. ‘Duration’ was determined from the 30 minute sand glasses used to set the sailors’ watches. The sequence of course changes was plotted on a chart to determine the gross determination of course and distance for the day.

Tasman measured from this how his longitude had changed since the previous day and added thius to the previous days longitude. This is what he recorded in his log.

Every step of this process included potential error, and these errors compounded. However, even if his estimate of course and direction had been perfect, this method of ‘dead reckoning’ still contained an inherent error. The difficulty is, that the speed and direction measured is not the actual speed and actual direction… it is the speed and direction relative to the body water they are sailing on.

The observations Tasman made took no account of any movement in the ocean, and the ocean is not generally stationary, it moves in currents. If Tasman was sailing into a current then he would overestimate their actual speed, if they sailed with the current he would underestimated it. Tasman was completely oblivious to any current in the ocean body, so his estimate of longitude took no account of this movement. Tasman’s journey from the from Batavia to the Mauritius took him along the South Equatorial Current… a current flowing to the west. This is why he encountered his destination two days earlier than he expected.

Tasman's chart side by side with a 

modern map

Tasman’s chart side by side with a modern map

Tasman’s longitude estimate is only to be relied on over short time periods; otherwise, as his daily errors compound, the position indicated becomes increasingly less representative.

Given Tasman’s lack of knowledge regarding his true position, it is remarkable that his chart is even recognisable as New Zealand. Yet, despite not knowing his longitude, it bears astonishing similarity to a modern map.

Tasman’s navigation: Part 1

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Gerritsz world map

Gerritsz world map, 1616 (click to enlarge)

Abel Tasman was travelling at a time when the Earth was properly understood to be a globe, and there was a standard method used to represent any position on the globe.

latitude and longitude

Lines of latitude and longitude

‘Longitude’, or how far east or west around the globe you were, was measured in degrees around a circle located on the equator. Lines of equal longitude continued directly north-south, to the poles. At the time Abel Tasman was voyaging, the Dutch convention was that all longitude was measured as East or West of the Peak of Tenerife. The Greenwich ‘meridian’, the line of 0° Longitude, wouldn’t be universally adopted until 1884.

Similarly, how far north or south you were, ‘Latitude’, was measured in degrees north or south of the equator, relative to the centre of the Earth.

This measurement system allowed any position on the globe to be represented unambiguously by the pair of numbers describing its latitude and longitude.

However, being able to accurately represent a location on the globe is quite different to working out the latitude and longitude of your current position… and Abel Tasman had only rudimentary methods for deriving this.

Tasman’s journal records his latitude and longitude, as well as his course directions, and bearings to visible features. In order to successfully interpret Tasman’s journal and charts it is important to understand just what he is measuring and recording.

COMPASS DIRECTIONS

Tasman reports course directions, and directions to land features using a 32 point compass. Most people are familiar with 16 point compass notation, but Tasman’s compass has a subdivision between each of those divisions with the additional direction indication ‘by’. The pattern of the compass used by Tasman is shown below.

lat and lon

Abel Tasman’s compass

His compass was a large and crude looking instrument suspended in a shape rather like a hanging flower basket. Inside that housing, the ‘card’ carrying the magnetic needle, was balanced on a needle. The top was sealed over with glass to keep it watertight.

Mounted around the top was a cross bar with sights on each end. These sights were used for lining up land features and also for getting accurate bearings of the sunrise and sunset positions.

All readings from a compass contain a discrepancy known as ‘variation’ which must be corrected. The ‘north seeking pole’ of the compass does not actually point quite to the true north.

The Earth’s magnetism is caused by the spinning ball of molten iron in the Earth’s core. The axis of that magnetic field is not perfectly in line with the axis that the Earth rotates around. The angular difference between the ‘true north’ position, and ‘magnetic north’ is called the ‘variation’. To compound matters, variation differs as you move around the globe, and different rock conditions cause local changes in the variation angle, as Tasman noted on Nov 22nd 1642;

‘we found that our compasses were not so steady as they should be, and supposed that possibly there might be mines of loadstone about here, our compasses sometimes varying 8 points from one moment to another’

To determine precisely where ‘true north’ was in comparison to his compass, Tasman used the sun. He measured the bearing of the rising sun, and that of the setting sun. True north lies directly between these positions (in the Southern hemisphere the sun still rises in the east and sets in the west, but it progresses there via the north). His variation on that day was the difference between what his sunrise/sunset bearings reported as north, and what his compass showed.

Tasman’s journal records all bearings as ‘true’ not ‘magnetic’; he had already made the correction for variation as he wrote his journal. On days that he was able to see the sunrise and sunset he also recorded the variation.

LATITUDE

How far north or south you are can be worked out from the knowing maximum height of the midday sun.

At the equator the midday sun is overhead, but at the poles it is only on the horizon. The further you move from the equator, the lower the angle of the midday sun. The angle it deviates from straight overhead is the angle you are away from the equator.

This is almost the correct answer, but like variation there’s another little twist. The Earth’s axis of spin isn’t quite upright in relation to its orbit around the sun. This is what causes summer and winter, and at any location on the globe, the midday sun is higher in summer than it is in winter.

There is a special correction chart for this, and Tasman carried one. It is a table showing the height angle, or ‘inclination’ of the midday sun at the equator for each day of the year. By comparing your measured midday inclination to the inclination at the equator for that day, you can derive your latitude.

As long as Tasman knew what day it was, and could measure the inclination of the midday sun, then he could work out his latitude.

Cross staff

Cross staff

In the fourteenth century Astronomers developed an instrument that did exactly this, and it quickly became an essential mariner’s tool. It was called a ‘cross staff’ among other names, and was a simple but remarkably effective device.

The cross staff had a long straight shaft, with another sliding bar mounted perpendicular to it in a way that allowed it to move up and down, like a trombone.

To use it you rested the end of the shaft on your cheek under your eye. You the slid the crossbar up and down, and raised the long bar until one tip of the crossbar was on the horizon, and the other tip on the sun.

The long bar of the cross staff was graduated with measurement marks. The position of the crossbar along the long shaft indicated the height of the sun in degrees above the horizon. Starting just before noon you measured the height of the sun, and kept measuring the inclination until it passed its peak. The highest angle recorded was the noon inclination.

It was a crude device, but it worked surprisingly well. Christopher Columbus used one of these to cross the Atlantic and return to the same Island in the Caribbean three times.

Tasman however, probably did not use one of these.

The cross staff was simple but using it had two major difficulties. You had to look at two places simultaneously; the horizon and the sun, and any movement of the long shaft as you did this introduced an error. The other issue was a practical one… you had to look directly at the sun.

Tasman’s journal records latitude to the minute. You can’t reliably make a measurement to the nearest minute of a degree with a cross staff, it’s simply too crude.

Hoekboog

Hoekboog, or back-staff

Willem Jansz Blaeu was a famous map maker. He prepared maps for the VOC as a contractor and was a close friend of Hessel Gerritsz. In 1625 he published this illustration of a ‘Hoekboog’, or ‘angle bow’. This was a more accurate instrument that overcame the issues of the cross staff. It had been in use by the Dutch since at least 1623.

With the Hoekboog you stood looking at the horizon with your back to the sun (hence the English name for this type of instrument, the ‘back-staff’).

The eyepiece ‘F’ slides up and down, on a graduated bar ‘D’. You look through the eyepiece at the flat plate ‘A’. This has a notch cut in it, which you look through and line up with the horizon. The piece ‘G’ has a sharp edge on it and casts a shadow from the sun behind onto plate ‘A’. By adjusting the eyepiece up and down the suns shadow is made to line up with the horizon. Then the ‘inclination’ angle is read off the graduated bar ‘D’.

In his journal Tasman recorded his daily latitude and this was remarkably accurate. The latitude he recorded for Cape Foulwind is only wrong by 8km. From his deck, an observer could see a 100m high cliff from 40km away.

The cross staff and the Hoekboog were both accurate enough to have the error in measured latitude smaller than the distance that you could see. They were accurate to get you within sight of your objective.

There was no similar instrument for measuring Longitude. Whilst mariners could reliably know how far north or south they were, they could not confidently asses their position east-west.

It was because of this that the common practice for travelling long distances across the open ocean (where there are no physical features to tell you where you are) was to sail ‘lines of latitude’.

On a long journey on the featureless ocean, mariners would manoeuvre themselves to the same latitude as their destination, and then hold their course on that latitude until their objective came into sight. Tasman’s journal shows that this is precisely what he did on his journey to the Mauritius.

Aug 17th.

‘it was resolved that from Sunda Strait we shall sail 200 miles to the south-west by west, as far as 14° South Latitude; from there to the west-south-west as far as 20° South Latitude, and from there due west as far as the island of Mauritius.’

Mauritius lies in the latitude 20°S, and Tasman could measure his latitude accurately enough to be within sight of features at a given latitude. So he moved to the latitude of the Mauritius, and continued on until his destination came into view.

He was able to confirm that he had reached his destination by consulting his collection of coastal surveyings. These were drawings showing what the coast looked like at specific places. If you were trying to find a given place, and were in the correct latitude, then when you saw land you compared it to what you saw in the drawing. If it was the same, then you had arrived.

COASTAL SURVEYS
On Tasman’s voyage they were expected to meet new lands, and in order to enable other mariners to find these places in the future they drew outlines of the coasts that they saw.

All VOC vessels had a ‘Merchant’ on board; the person in charge of buying goods to trade. On Tasman’s voyage, the Merchant was carefully chosen for his additional skills. They appointed… ‘Jsaack Gilsemans, who is sufficiently versed in navigation and the drawing-up of land-surveyings’.

The illustrations in Abel Tasman’s journal were drawn by Isaac Gilsemans, from the deck of the Zeehaen… and these illustrations included coastal surveyings.

When they discovered new land in the Southern Ocean they named it ‘Anytony van Diemens Landt’ after the Governor of Batavia. Gilsemans recorded the outline of this land so that subsequent mariners could correctly identify the location.

When you have reached the West Coast of Tasmania, at the Latitude of 42° 30′ you will see this.

Tasmania coastal survey

Tasmanian coastal drawing by Isaac Gilsemans

The captions read:
A view of the coast when you are 5 myles from it.
A view of the coast when you are 2 myles from it.
A view of Anthonij van Diemens Landt, when you come from the west, and are in 42½ S. Latitude.

Without knowledge of longitude, coastal surveys were an essential part of navigation. When a mariner had located land at the specified latitude he could verify his position by comparing what he could see, with the coastal survey. If he saw the same in the drawing that he saw from the deck, then he knew precisely where he was… and he could then correct his longitude accordingly.

As Abel Tasman approached Mauritius his reported longitude was in error by 670 km, but it was nearly correct when he left. The Dutch navigators used the coastal surveyings to re-set their longitude.

Tasman had no instrument for measuring longitude, yet for his noon position he reported both latitude AND longitude. So how was this done?