Saturday, March 10, 2012

30 dolphins stranding and incredibly saved

About 30 dolphins stranded and saved by local people at Arraial do Cabo (Brazil)

in the morning at 8:00 AM on March 5th 2012.
>>> geolocalization with the Marine GeoGarage <<<
The whole episode was captured by Gerd Traue in a video (above),
which has racked up over a million views online.
They were apparently caught in a strong ocean current.

From NewScientist

It was just another day on the beach for holidaymakers off the coast of Rio de Janeiro, Brazil, when a pod of about 30 dolphins swam ashore on Monday.
Beachgoers quickly came to the rescue, rushing into the sea and dragging the dolphins by their fins and tails into deeper water.

What can experts learn from the footage?
The species involved, for one.
These are common dolphins (Delphinus delphis), which typically live a long way off shore, says Mark Simmonds of the Whale and Dolphin Conservation Society, a global charity.

However the video does not reveal what caused the stranding – fishing boats or sonar are two possibilities.

Had experts rescued the dolphins, says Simmonds, they may have examined the individuals for damage, such as net marks, that may have provided clues.
But he says the dolphins in the video appear to be healthy.

Out of their depth

The topography of the coastline may have disoriented the dolphins, says Michael Moore of the Wood's Hole Oceanographic Institution in Massachusetts.
It would not be surprising if offshore dolphins like these had trouble navigating the sandbars and silty seabeds found in shallow waters.

Nor is it surprising that such a large number of dolphins would head for land together.
Dolphins are social creatures, so it would take only one member of the pod to go astray – say, if it was diseased – and the others would follow.

This social behaviour is what makes mass strandings of cetaceans so common.
In the last month, for example, an unusually high number of dolphinsreportedly 179 – has been stranded in Cape Cod, Massachusetts.

Call in the experts

Understanding what's going on in such cases is especially hard because scientists must rely largely on postmortem evidence.
Moore and others are trying to develop early warning systems to get to the dolphins while they're still alive.

Sensors deployed in regions where strandings are common, for example, could detect the sounds of the cetaceans nearing shore and send a text message warning officials of an imminent stranding.

The Brazilian video has widely been greeted as a good news story.
But Simmonds is uneasy about this.
"There's a clock ticking, so it's important to respond quickly, but it's also important to move them in the right ways," he says.
Typically it's best not to touch the animals and to call in experts instead, he says.
"Pulling their flippers can dislocate their bones, or even pull a flipper right off."

Friday, March 9, 2012

Possible reasons identified for Rena grounding

Part of chart NZ541 showing passage plan, plotted positions and voyage data recorder GPS positions on approach to Tauranga
>>> geolocalization with the
Marine GeoGarage <<<

From NewsTalkZB

Taking shortcuts and miscommunication have been identified as possible reasons for the Rena hitting Astrolabe Reef.
The Transport Accident Investigation Commission (TAIC) has just released its interim report into the accident.

It shows the second-mate began taking the ship closer to the reef at 1.20am because it was running late.
Investigator in charge Robert Thompson says the second-mate says he marked a point one nautical mile north of the reef, so he'd know to change course.
"The second-mate said he showed the master the charts and pointed out the amended passage plan including the mark north of Astrolabe Reef. The master said he did not study the charts. And the mark was put there after the grounding."

Part of chart NZ56 showing passage plan, plotted positions and GPS positions
obtained from the voyage data recorder on passage around Mahia Peninsula

>>> geolocalization with the Marine GeoGarage <<<

Mr Thompson also says the 2am position plot was added after the grounding, and was north of the GPS tracking of the ship's actual position.
"He was not able to access the chart because the master and his second mate were leaning over the chart. The position was not plotted on the chart until after the Rena had run aground, and then it was plotted on the chart north of the vessel's actual 2am position."

Annotated screenshot of the Rena's radar (colour reversed for clarity)

Mr Thompson says an intermittent echo on the radar was noticed nine minutes before they ran aground.
"But it's also not uncommon to have false echoes and the captain's reaction in this case will be the subject of further analysis."

He says there's no evidence the master had been drinking.

Mr Thompson says they misread the ship's position from both the gyro compass and the GPS data.
"Under the influence of tide and current and wind and waves and including any compass error, the ground track was two degrees to the south of the vessel's heading."

Mr Thompson also says chart plots at 1am and 1.20am were put on different charts and weren't cross referenced against each other.

Transport Minister Gerry Brownlee says the report establishes some verified facts that will enlighten ongoing investigations.

Analysis, findings and recommendations stemming from the Rena incident will be subject to the Commission's final report due next year.
Meanwhile the Bay of Plenty Regional Council says today's report gives some clarity about how the accident happened.

Council chairman John Cronin says he hopes the report will help the community in the healing process.
He says they'll continue to work to restore their environment and help the community and iwi recover.

However Labour's deputy Leader Grant Robertson says while the series of mistakes identified are important, investigating the adequateness of the Government's response is also needed.
"But really a thorough investigation into the response, into how people were informed, what kind of communication was there with communities and what we can learn from this is what's really important."

But Minister of Transport Gerry Brownlee indicates that won't happen.
"We have, as you know, two inquiries that are going on plus a court case so it's not exactly as if this is being pushed to one side."

Links :
  • NZ Herald / StuffNZ : Rena report: Crew took shortcuts
  • SunLive : Rena report points to human error
  • TheTelegraph : 'New Zealand's worst maritime disaster caused by ship taking short cut'

Thursday, March 8, 2012

Protecting the world’s coral reefs through mapping

Take a tour of coral reefs around the world with the World Resources Institute's Reefs at Risk project and Google Earth.
The journey to each of the six coral reef regions provides an overview of the biodiversity of reef ecosystems, their importance to people and local economies, and the types and magnitudes of threats that reefs face, illustrated with vibrant underwater footage of coral reefs and photos of activities that influence reef condition.

From Google

The Reefs at Risk project raises awareness of threats to coral reefs and provides information and tools to manage coastal habitats more effectively.

Since 1998, the World Resources Institute (WRI) has been using GIS (Geographic Information System) models to develop map-based assessments of threats to the world’s coral reefs.
Reefs at Risk Revisited, released in February 2011, is the latest assessment in the series and is based on a nearly three-year study that produced the most highly-detailed global maps of coral reef threats to date.
The study analyzed and mapped threats to coral reefs from local human activities such as coastal development, unsustainable fishing, and marine and land-based pollution, as well as climate-related threats caused by increasing levels of greenhouse gases in the atmosphere.

While the Reefs at Risk Revisited report, maps and data sets are the core components of our work, we found ourselves still searching for a compelling way to convey our findings on these dynamic yet fragile reef ecosystems.
We also wanted to generate greater awareness of the unique characteristics of reef habitats across different geographies and the irreplaceable cultural and life-sustaining services that reefs provide to people all over the world.
All of these elements are best communicated when you can see them for yourself, which is why we created a virtual tour of these reefs around the world with our Google Earth Outreach Developer Grant.

In the tour, the Reefs at Risk Revisited maps come alive on Google Earth with photos and underwater video from each of the major coral reef regions of the world: the Caribbean, Middle East, Indian Ocean, Southeast Asia, Australia, and Pacific, as well as a global-level introduction and conclusion.
The journey to each region provides a brief overview of the biodiversity of reef ecosystems, their importance to people and local economies, and the types and magnitudes of threats that reefs face, illustrated with footage of both healthy and damaged reefs.

You’ll also experience first hand these fantastically dynamic and productive ecosystems that extend across about 250,000 sq km (96,500 sq miles) of the tropical ocean.
Even though this area represents less than 0.1% of the global ocean, reefs are home to as many as 25% of all known marine species.
In the tropics, coral reefs are not only a critical habitat for marine species, but they also play an essential role in the lives of millions of people who live near them: they provide food and income from fisheries, revenue from tourism, and protection for coastal communities from storm surges.

Coral reefs are classified by estimated present threat from local human activities :
overfishing and destructive fishing, coastal development, watershed-based pollution, and marine-based pollution and damage

Among the other products of the Reefs at Risk Revisited project are global maps of coral reefs rated according to level of threat in the present, 2030, and 2050.
These maps are available as downloadable KML files on the WRI website for viewing on Google Earth, and also as part of an online map developed using Google Fusion Tables.
With these interactive maps you can zoom in to your favorite reef to explore it more closely in your own self-guided tour.

We hope that you enjoy our tour and maps, and that you are able to visit a coral reef to learn more about these important and unique ecosystems.
With improved understanding, we can manage and protect these resources so that we can all enjoy them and benefit from them for generations to come.

Wednesday, March 7, 2012

Restored map reveals early Arabian trade links with China

What is the Selden Map? A late Ming water color map of East Asia, including China, Korea, Japan, the Philippines, Indonesia, Southeast Asia and part of India, probably executed in the 1620s.
The map has no title, and is very large, approximately 1×1.5m.
The text is in Chinese, but there are some Latin annotations by a later hand.
The map shows shipping routes and compass bearings from the port of Quanzhou across the entire region.
A panel of text on the left of the map near Calicut, its western extremity, gives directions of the routes to Aden, Oman, and the Strait of Hormuz.
The map has always been known as an interesting curiosity from the time it arrived in the ‘Bodleian’ Library, but its importance was first recognized by the visiting American scholar Robert Bachelor in January 2008.
He was the first to notice the shipping routes, which make the map unique among both Chinese and indeed European maps of the period, and has described it as “an object of globally recognizable significance.”

From TheNational

With Beijing's claims to the South China Sea never far from the headlines thanks to the assertiveness of modern China, a recently restored map offers some welcome historical perspective on the oceans in East and South East Asia.

Unearthed nearly four years ago in a fragile and discoloured state, the Selden Map of China was bequeathed to England's University of Oxford in 1659 after the death of John Selden, a London lawyer and linguist.

It shows sea routes fanning out from Quanzhou in Fujian province in south-east China, a city that attracted a mention in Marco Polo's account of his travels, and which still contains reminders of its Muslim-influenced past.
Indeed, while it remains unclear exactly who produced the map, one possibility is that a member of Quanzhou's sinicised community of Islamic merchants was responsible.
Such is the interest in the map's origins now, that Timothy Brook, a professor at the University of British Columbia and author of several books about Chinese history, is currently writing a volume titled Who Drew the Selden Map?.

"Rather than showing China from within, it sees China from without. In particular, it sees it from the water, which is not the perspective that Chinese themselves have taken when looking at their country," says Brook.

China is depicted in terms of its relationship with its neighbours, and the map even stretches to include parts of India.

"What this map shows is an interest in the fact that Chinese (and especially Fujianese) people lived in most of the major ports in South East Asia by the early 17th century, from those of Siam, the Malay Peninsula and western Java to Manila and Nagasaki," says Robert Batchelor, an associate professor of history at Georgia Southern University.

The document also indicates an interest in the Arabian Peninsula among the Fujianese merchants.
One corner has written directions for travelling from Calicut in India to Aden in Yemen, Salalah in Oman and the former kingdom of Ormuz in the Arabian Gulf.
This ties in with the possibility that the map may have been produced by a group of Quanzhou-based Islamic merchants.
"It also suggests that the ties between Arabia and South China were [neither] superficial, [nor did they] disappear with the arrival of Europeans in the 16th century," according to Batchelor. "That is why this is a map that should be of interest in particular in Iran, the UAE, Oman and Yemen. It speaks of a continuity of global history."

Selden himself wrote a book titled Mare Clausum (Closed Seas) that argued against the Dutch view at the time that favoured freedom of the seas and, says Batchelor, the lawyer read the map as indicating a kind of dominion over the areas where trade was taking place.

However, the merchants of the early 17th century are unlikely to have seen it as a way of staking claims to, for example, mineral or fishing rights.
Instead, Batchelor takes the view it shows their interest in and involvement in the seas depicted.
"Rather than a closed China or a globally overpowering China, the Selden Map gives us a window onto the complex and dynamic set of relations that defined East Asian trade in the 17th century," he says.

Brook says there is nothing to show Ming China tried to claim territory beyond its borders.
The map is one of trade, not empire: "Anyone who tries to use this map to enlarge Chinese claims for a greater empire is engaging in political mischief," he says.

Yet, while the map may not have great political significance, even in historical terms, it is of interest to academics, not least because it offers information available from relatively few other sources.
Batchelor contrasts this situation with that of the English East India Company and its Dutch equivalent.
Their links to their respective governments meant the activities of these organisations were well documented and the evidence preserved for later generations.

"By comparison, we know very little about 17th-century Chinese merchants because they were not sponsored by the state. Their private archives and libraries have largely been destroyed over the centuries," he says.

It is perhaps especially important that sources such as the Selden Map are understood, given that analysis of certain documents has often resulted in outlandish claims about Chinese explorers.
In particular, the author Gavin Menzies suggested in his book 1421: The Year China Discovered the World that the 15th-century Chinese admiral Zheng He discovered the Americas before Christopher Columbus.
The claim is based upon an 18th-century map said to be a copy of one from the 15th century.
Menzies' views are generally dismissed by academics and Batchelor describes such "dubious" maps as "efforts from later centuries to make up for the lack of a Selden Map".

Clumsy preservation efforts meant that, until recently, the map was in a poor state of repair.
It was discoloured, the paint on it was cracking and a linen backing from a century ago had turned stiff, cracked, and been patched up.
Restoration took about a year, says Robert Minte, a conservator at Oxford's Bodleian Library, who carried out much of the work.
Experts at the British Library and the British Museum also took part.

"It was in an extremely fragile condition," says Minte.
"It had been [lined] with very heavy cotton and rolled up very tightly, so every time the map was unrolled there were fragments coming off. It was very difficult to handle."

The task was to remove the heavy cotton lining and the various patches while trying not to damage the "extremely thin" Chinese paper on which the map was painted.
This work, much of it done with the map face down on a Perspex table, was complicated by the fact that many of the patches were stuck on with a strong glue and the paper to which they were attached was fragile.
A bamboo spatula was used to tease away the patches.
New pieces of Chinese paper were added to fill in gaps in the original map, and three linings of Japanese paper were attached to strengthen the map, although Minte describes it as "still quite vulnerable".
"It can [however] survive far more safely and be displayed much more safely," he says.

The painstaking restoration achieved more than helping to preserve the map.
It also uncovered many features that would otherwise have been hidden.
"Because we spent so many weeks and months looking at the map, we could describe areas of detail through the work we were doing. Some of the most interesting were the details on the back underneath the cotton lining," Minte says.

For example, some trading routes are mapped out on the back of the map, in particular the main trade route from Japan to Vietnam, suggesting that whoever produced the map started on one side and then may have decided, for whatever reason, to turn the paper over and have another go.
"We're not quite sure whether that's a preliminary drawing or something they started, then made a mistake and changed something," Minte says.

Batchelor thinks the drafts also indicate a fundamental difference in the technique of mapmaking, with the routes drawn first, in contrast to western maps based on a grid system.
"This suggests different approaches to mapping, navigation and mathematics, which have interesting similarities to our current notions of 'networks'," he says.
"Indeed, the map itself shows a system rather than just routes, and this is very early for anyone to be thinking about trade systematically."

Tuesday, March 6, 2012

America’s maritime power

Bordered by ten nations and including some of the world's most important shipping lanes and fisheries, the South China Sea is a vital region.
Critically important mineral resources, including oil, are thought to be there in large quantities as well.
The Chinese have long laid claim to nearly the entire South China Sea.

From LeMondeDiplomatique

US returns in strength to the Pacific Now that the US is substantially reducing its military spending and withdrawing from present wars, its future intentions, and those places it will seek to control, are becoming clear.
The most important will be the Pacific and the South China Sea.

“Our nation is at a moment of transition,” said President Barack Obama on 5 January when he unveiled a new national defence strategy.
This means the size of the US military will be reduced and some combat missions curtailed, notably mechanised ground combat in Europe and counterinsurgency in Southwest Asia.
The aim is to focus more on other parts of the world, especially Southeast Asia and the Pacific, and on other objectives: cyber warfare, special operations and sea control.
“The US joint force will be smaller, and it will be leaner,” said defence secretary Leon E Panetta. “But…it will be more agile, more flexible, ready to deploy quickly, innovative, and technologically advanced”.

According to Obama and Panetta, the strategy reflects altered circumstances at home and abroad.
The US, weakened by the economic crisis, has a ballooning national debt.
The department of defence must make spending cuts of $487bn over the next 10 years to comply with the 2011 Budget Control Act; and more cuts are possible if Congress fails to reach agreement on additional budget-saving measures in the months ahead.
Abroad, military pressures are not decreasing despite the withdrawal from Iraq, and eventual escape from Afghanistan: the US faces new threats of potential conflict, for instance with Iran (see Iranian options) and North Korea, plus the growing spectre of a rising China.

At first glance the new defence policy can be seen as a pragmatic response to altered fiscal and geopolitical conditions, aimed at providing a smaller force with greater capacity to confront future dangers.
On closer inspection, one can discern a larger strategic intent. Faced with the inevitable erosion of its status as sole superpower and the rise of ambitious rivals in Asia, the US seeks to perpetuate its global primacy by maintaining superiority in key areas of the world and critical forms of combat.
In particular, it will aim to dominate the maritime edge of Asia, in an arc from the Persian Gulf and Indian Ocean to the South China Sea and northwest Pacific.
This will require the preservation of US superiority in air and naval warfare, and continued dominance in cyber-warfare, space technology and other specialised fields.
Counter-terrorism will remain an important Pentagon function, but will be largely delegated to highly trained Special Forces equipped with killer drones and other high-tech paraphernalia.

Managing the contraction of overseas interests and commitments — or, as some would have it, managing the decline of empire — is never easy.
Other great powers that have had to undertake such endeavours — Britain and France after the second world war, Russia after the collapse of the Soviet Union — have found it exceedingly difficult.
Often they have embarked on ill-advised military adventures, such as the 1956 Anglo-French invasion of Egypt (Suez) and the 1979 Soviet invasion of Afghanistan — actions that hastened the collapse of empire, rather than delaying it.
When the US invaded Iraq in 2003, it was at the peak of its power; but the ensuing insurgency lasted so long and cost so much — an estimated $3 trillion — that the US has lost the will (and much of its capacity) to fight any new protracted ground wars in Asia.
From here on, it is highly unlikely that Obama or any other American president, Democrat or Republican, would authorise a major operation akin to the interventions in Iraq and Afghanistan.

Obama and his top advisers, cognisant of this history, are determined to avoid the strategic mistakes of earlier leaders.
But if they recognise the folly of attempting to cling to all overseas commitments, knowing it would bankrupt the nation, they have no intention of presiding over a rapid contraction of foreign interests, seeing this as recipe for greater chaos and decline.
Instead, they are seeking a middle way, choosing to reduce US commitments in some areas — Europe, in particular — while bolstering the nation’s capacity to prevail in areas deemed most important for America’s continued global supremacy.

Containing China

This means dominating the western Pacific and containing Chinese power.
“In many respects, the broader Pacific will be the most dynamic and significant part of the world for American interests for many decades to come,” said Deputy Secretary of State William J Burns last November.
“It already includes more than half of the world’s population, many of its most important economies, key allies, and emerging powers.”
For America to remain strong and prosperous, Burns indicated, it must concentrate its energies in this area and ensure that China does not gain power and influence to America’s disadvantage. “As Asia undergoes profound changes, we need to develop the diplomatic, economic, and security architecture that can keep pace”.

This new “architecture” has many dimensions, military and not.
On the diplomatic front, Washington has bolstered its ties with Indonesia, the Philippines and Vietnam, and reinstated formal relations with Burma.
The White House is also seeking to invigorate US trade with Asia, and pushing for the establishment of a regional trade pact, the Trans-Pacific Partnership (TPP).
This is implicitly aimed at countering the rise of China and its influence in Southeast Asia.
By reinstating ties with Burma, for example, the US gains a voice in a country where China, until recently, had few competitors; the proposed TPP would exclude China on technical grounds.

Alongside these economic and diplomatic moves are significant military initiatives.
For Asian states to grow and prosper, American strategists believe, they must enjoy unhindered access to the Pacific and Indian Oceans (along with connecting waterways such as the Straits of Malacca and the South China Sea) in order to import essential raw materials (especially oil) and export manufactured goods.
As Burns explained in November, “Asia’s rise has been so dramatic that it is not just remaking Asia’s cities and economies — it is redrawing the geostrategic map. To give one example, half the world’s merchant tonnage now passes through the South China Sea.”

By establishing naval dominance in the South China Sea and adjacent waterways, the US could exercise a form of latent coercive power over China and the other states in the region, much as the British navy once did.
American naval strategists have long been arguing for such a stance, claiming that America’s singular advantage lies in its ability to control the world’s major sea-lanes — an advantage enjoyed by no other power.
It now appears as if the Obama administration has embraced this outlook.
This was clearly implied in the moves Obama announced during his visit to the region in November.
In spite of budget cuts, he said in Canberra: “We will allocate the resources necessary to maintain our strong military presence in this region” and will be “enhancing our presence in Southeast Asia.”
This will involve more frequent deployments by US warships and military exercises in the region.
In addition, Obama announced the establishment of a new US military base at Darwin, on Australia’s north coast, and increased military aid to Indonesia.

Presence and deterrence

Implementation of this grand geopolitical vision has obvious implications for the development of military policy, and this is clearly reflected in the strategic policy unveiled by Obama and Panetta in January.
“As I made clear in Australia,” Obama said, “we will be strengthening our presence in the Asia-Pacific [region], and budget restrictions will not come at the expense of that critical region.” Panetta added: “The US military will increase its institutional weight and focus on enhanced presence, power projection, and deterrence in the Asia-Pacific”.

Although the policy document itself does not identify which specific military components will be favoured, it is clear that emphasis will be placed on naval forces — especially aircraft carrier battle groups — as well as advanced aircraft and missiles.
Thus, while the US army will see a reduction in its total strength from approximately 570,000 troops today to 490,000 in 10 years’ time, Obama has vetoed plans for any reduction in the navy’s carrier fleet.
Also, the US will invest substantially in weapons aimed at defeating potential adversaries’ “anti-access/area denial” (known as A2/AD) capabilities — the planes, missiles, and ships designed to overpower US attack forces (especially aircraft carriers) in contested areas.
Because China is expected to enhance its capacity to strike American naval forces operating in the South China Sea and other areas on its periphery, US forces will be equipped with greater defences against these so-called A2/AD capabilities.

As the new Pentagon blueprint puts it: “In order to credibly deter potential adversaries and to prevent them from achieving their objectives, the United States must maintain its ability to project power in areas in which our access and freedom to operate are challenged” — a clear reference to the East and South China Seas, as well as waters off Iran and North Korea. In these areas, it is claimed, potential adversaries “such as China” will use “asymmetric means” — submarines, anti-ship missiles, cyber warfare — to defeat or immobilise US forces.
Accordingly, “the US military will invest as required to ensure its ability to operate in anti-access and area denial (A2/AD) environments”.
This means that the US will place top priority on dominating the maritime periphery of Asia, even in the face of opposition from China and other rising powers.

Links :

Monday, March 5, 2012

Gerardus Mercator : father of modern mapmaking

Today March 5, 2012 marks the fitfth centenary of the birth of Gerardus Mercator who developed the famous map projection able to represent lines of constant course as straight segments, thus preserving the angles, which proved useful in navigation.

The 'Mercator projection' that bears his name was first used by him in 1569 for a wall map of the world on 18 separate sheets entitled:
"New and more complete representation of the terrestrial globe properly adapted for its use in navigation."
The 'Mercator projection' had the property that lines of longitude, latitude and rhumb lines all appeared as straight lines on the map.
He had, he wrote on the map:
"... spread on a plane the surface of a sphere in such a way that the positions of all places shall correspond on all sides with each other both in so far as true direction and distance are concerned and as concerns true longitudes and latitudes."

With his world map published in 1569, the title of the original map telling us as much (Nova et aucta orbis terrae description ad usum navigantium emendate et accomodata translated as “new and improved description of the world amended and intended for the use of navigators”), the Flemish cartographer presented a new method to portray the globe on a two-dimensional, angle-preserving map.
This means that angles between the different directions on the map correspond to the directions on the earth’s surface.

On this Mercator projection, Greenland and South America appear similar in size.
The inset map shows that South America is actually about 15 times larger than Greenland.

(see "political controversy")

As it is impossible to project the spherical surface of the earth on a flat surface without distortion, Mercator’s cylindrical map projection uses these distortions to render them user-friendly for nautical purposes.

Marine GeoGarage for nautical charts ideally based on the Google Maps web viewer

Although the Mercator projection significantly distorts scale and area (particularly near the poles), it has two important properties that outweigh the scale distortion:
  • it’s a conformal projection, which means that it preserves the shape of relatively small objects. This is especially important when showing aerial imagery, because we want to avoid distorting the shape of buildings (square buildings should appear square, not rectangular).
  • it’s a cylindrical projection, which means that north and south are always straight up and down, and west and east are always straight left and right.
Graphic scale from a Mercator projection world map, showing the change with latitude - 
A good illustration of how distorted the Mercator projection really is
 Google Maps is based on a close variant of the Mercator projection : to simplify the calculations, Google Maps uses the spherical form of this projection also called WGS84 web Mercator (or pseudo-Mercator), not the ellipsoidal form.
Since the projection is mainly used for map display, and not for displaying numeric coordinates (the Latitude/Longitude geographic coordinates of features on Google Maps are the GPS coordinates based on the WGS 84 datum), we don’t need the extra precision of an ellipsoidal projection.

The difference between a sphere and the WGS 84 ellipsoid causes the resultant projection not to be precisely conformal.
However, the discrepancy is meaningless at the global scale but causes maps of local areas to deviate slightly from true ellipsoidal Mercator maps at the same scale.
However, the spherical projection causes approximately 0.33% scale distortion in the Y direction, which is not visually noticeable.

Since the Mercator projection goes to infinity at the poles, it doesn’t actually show the entire world : so Google Maps cannot show the poles.
Using a square aspect ratio for the map, the maximum latitude shown is approximately 85.05 degrees north and south.
Although cutting off coverage, this is not considered a limitation, given the purpose of the service for aid to navigation.
Not a lot of boats sail at those latitudes...

The great circle route from Seattle to London is a straight line on the gnomonic map projection.
Circular “compass roses” at points where the route crosses major meridians are projected as ellipses, showing the distortion of directions that makes it possible to measure true azimuths only roughly along the route.

(courtesy of ESRI)

Portion of a world map made with the Mercator projection showing the rhumb line and great circle route from Seattle to London.
The great circle route, obtained from a gnomonic projection, has been divided into 500 nautical mile legs.
Since the Mercator projection is conformal, each simplified compass rose is correctly projected as a circle.

(courtesy of ESRI)

Originally designed for use in nautical navigation, Mercator's projection is a map of the Earth's surface on which straight lines, called rhumb lines (or loxodromes), represent a course of constant compass heading.
On the surface of a sphere, however, such rhumb lines are not the shortest distance between two points. Rather, the shortest distance between two points is a great circle (or orthodromes), whose shape is displayed for the routes on the Marine GeoGarage charts (great circle distance calculations)

Links :

Sunday, March 4, 2012

VOR at reaching

Bringing the colour: Cammas stand out as French crew led on way to Auckland

Credit: Yann Riou/Groupama Sailing Team/Volvo Ocean Race

Live at the extreme in the Volvo Ocean Race
(other video)