I'm going to talk about an Internet-application I developed to georeference scanned historical maps. This is what it looks like: left a cadastral map from 1832 of the beautiful village of Eijsden in the extreme south of the Netherlands, right a modern reference map of that area, including Maastricht, the city of the Treaty that created the European Union in 1992. I will illustrate the program with examples from this region, my native ground, Dutch South Limburg, on the borders between Holland, Belgium and Germany. Then I'll tell something about the technology: how to integrate mapping Internet servers, database-systems and browser-based user interfaces by means of AJAX-based technologies. Finally I'll give some indications of further developments, made possible with these maps.

So let's start with the region: it is situated in a place called Europe, which is about half way between the United States and Russia. It is the border region between the Netherlands, Belgium and Germany. We call it the Euregio, because it's really international, with very diverse identities. You can even see this on the satellite image: the land really changes color when you cross the borders! Here are the flags: in the middle the European Union, to the right and left Belgium and Germany, and at the corners Holland and the Walloon cock, the symbol for the French speaking part of Belgium. For some reason they forgot the Flemish flag. You can see that this is Belgium, because they still have electricity above ground there. People talk some sort of Dutch in Maastricht (Mestreechs), some sort of German in Aachen (Öcher Platt), and some sort of French in Liège (Walon). They even sometimes forget their English: this poster: means "appartment for rent" in three languages. Those national boundaries are something imposed upon us in 1815, but even after that year it remained a very open country, until 1914, when the borders were at last rigorously closed. Fortunately, it is slowly regaining its former freedom nowadays.

Apart from the magnificent historical cities of Maastricht, Aachen and Liège, all three chock-full of medieval and post-medieval monuments, the country is quite rural. For those of you who know Tolkien's "the Lord of the Rings": this really is the Shire, very green and hilly, with villages, cottages, brooks, and cows, lots of cows. And of course: hobbits. These are the ax-men: they walk before the yearly sacramental procession, to remove obstacles thrown here by people who don't like processions, sometimes with the help of casual passers-by. These are very old traditions, by the way; this particular procession dates from the late Middle Ages, the Fête Dieu, instituted in Liège in 1345, and this May-Dance afterwards, in the villages of Oost en Maarland, is already mentioned in 1650 as being a very old institution, together with its organisers, the Jonkheid: the captain, lieutenant and flag bearer of the village. To be honest, the particular hobbits from that region don't really look quite like that: their beards are false, and most of those people look almost like you and me, like this couple sitting before their house. And it is about this house that I am going to speak, because that is where I was born and grew up, and these are my parents of course.

Here it is on Google satellite, south of Maastricht, along the river Meuse (this is from my dissertation back in the eighties. It's a bit rough, but better than Google). This is the village of Oost nowadays and in 1832. Here is the building I mentioned, on the 1832 cadastral map. You see here the castle of Oost, dating from the Middle Ages (That's how it looks nowadays, mark the red fence made from iron left over by the American army in 1944 ). I regret to say, though, that I was not born there. Actually, I was born on the farm opposite, which in this form dates from 1850, but has much older parts, probably late-medieval (the ivy-covered side at the left with the white stones behind it). It served for a long time as a horse-stable for the castle (note the circle of bricks before the gate on the left, to prevent the entrance getting too muddy), until my grandfather leased it in 1925 and started his own farm there.

Now: how are we going to georeference this scanned 1832 cadastral map? There are ten reference maps available at the moment; here you see the cadastral map of 2005 (note how similar it still looks to that of 1832), but we can also choose a recent topographical map. As you can see, it is possible to zoom independently on the scan on the left and on the map on the right. Also available is an older topographical map from about 1930 and even Google. We zoom to the castle (note that there is something funny with the location of our farm opposite the castle), shift-click, and point to similar points in scan and map, in this case the south-west corner of the castle (note the scan and map-coordinates at the top). When we have done this for at least three points, there is enough information available to georeference the map.

Note that if we add more points, Root-Mean Square Errors will appear (the two columns on the right). Anyway, it is now possible to position the scan on a map, so with the arrows at the bottom, we can change the extent of one window to that of the other. And with the button in the middle, the one marked 2, a cursor can be dragged in one window, showing the exact position of that spot in the other map. This works even if the maps have different scales or orientation

Actually there is something funny with the 2005 cadastral map. I put the control points on the old cadastral map at the corners of the castle. On the topographical map, the points are more than 10 meters distant from the castle. If the cadastral map really has this kind of error, we can expect lots of troubles arising from conflicts about old property boundaries, as these are often decided with the help of the modern cadastral map.

Note that we still haven't made any changes to the scan: all computations are done in the browser in real time by JavaScript. Once we push the G-button, the scan is transformed in a georeferenced Tiff-image (by gdalwarp) and can be shown oriented to the North and in the present Dutch coordinate system, as shown at the right side. The program has an option to use regular transformations or rubber-sheeting wit Gdal's thin plate spline algorithm. This means that we can also use an already georeferenced historical scan to georeference a neighbouring cadastral plan. For this kind of maps this is very important, as they are not measured error-free and have to be rubber-sheeted, some extensively. That way it is reasonably easy to get exact overlaps, e.g. with border roads on several cadastral plans.

Once the cadastral basemap is finished, every historical map can be georeferenced on the basis of this cadastral map, and there are lots of historical maps of this region. It has been a battlefield for all European wars from 1500 on and all wars need maps. At that time the great nation states started to develop, and the big players were England France, Prussia, Austria and the Dutch Republic. The big difference with the kingdoms and principalities of the Middle Ages was that these states fought their wars with mercenaries, and in that way centralized states with money could dominate decentralized regions, like the Euregio territories, which were mostly easy-going, not to say slow, ecclesiastical principalities, where the living was good because they were so fertile. That is why the small Dutch Republic and the not much bigger England were so important in international politics: their overseas trade and colonial empires furnished enough money for large standing armies, consisting of mercenaries from the poorer parts of Europe like Switzerland. You can see on the map that the Euregio lies somewhere in the middle of this wolf's den, and it never had a centralized government to keep the predators away. Moreover, it was very fertile, so foreign armies loved to stay there and bleed it white. Worst of all, it offered the best connection for armies traveling from south to north (France to the Republic), or east to west (Germany to France).

It all started with the Dutch War of Independence, nominally a war for religious freedom, in reality a war about taxes and money and naked power. This is Maastricht in 1568, and you see at the bottom the protestant army of the Prince of Orange (German mercenaries), and at the top the catholic army of the Duke of Alba (Spanish mercenaries). How small the part was that religion played, even so early in the war, can be seen from the fact that the catholic, Spanish Duke of Alba was excommunicated by the even more catholic bishop of Roermond, because of the exorbitant means the Spanish tax-gatherers used to collect their money. The bishop, by the way, had only been appointed by the most Christian King of Spain, Alba's master, a few years before. Some ten years later, the Spanish beleaguered Maastricht and took it for the Catholic cause, causing thousands of civilian deaths in and around the city. In 1632 the horror was repeated from the Protestant north by the Dutch Prince of Orange, a son of the father of the Dutch fatherland you met above (Oost is just visible at the right of the map), and in 1673 the Sun King, Louis XIV, arrived in all his glory from Versailles to lead the assault on Maastricht in the war that the rest of Europe saw fit to wage on the Dutch. The beleaguering of 1748 by the French Marshall of Saxe in the War of the Austrian Succession was equally an event of European magnitude. This war was, as its name said, about who was to be Emperor of Austria.

The first detailed, although very faulty, map of Oost dates from that last war. And those were only the big assaults; there were about ten more in that period, in the several Wars of Succession, the Nine Year's war, the Seven Year's War (even contemporaries couldn't tell any more what they were really fighting about). And even that was perhaps not the worst. We imagine war as soldiers killing people, but in reality not that many civilians were actually killed by soldiers during that period. People died like flies from the epidemics that followed large armies: dysentery, fever, etc, much like the Indians died in the new world. And those who survived had to pay. To survive all this, these people really must have been something like hobbits: sturdy in a inconspicous way.

In the eighteenth century, the armies of the nation states found a more efficient way of parasitizing on these regions than plundering and killing: taxes. These are the remaining accounts of taxes paid by the village of Oost, about 25 houses in that period, to foreign armies in the 17th and 18th centuries. Each paper is a different year; there are about twenty if them, not every tax-account has been preserved, and in the worst years, like the assaults on Maastricht, they didn't have the time to write everything down; they just got plundered.

How this worked in practice, you can see here: this is a "Sauvegarde" from the Prince of Tilly, general of the Austrian army in the War of the Austrian Succession in 1702 for the barones of Hoensbroek, the "Seigneur" of the village at that time who inhabited the castle, and the inhabitants of Oost, ordering troops not to molest the village. This protection had to be paid for of course, essentially a form of state-supported Mafia extortion, and please notice that Oost wasn't the only village that had to pay this "pizzo" money: the Sauvegarde is a printed form, with the name of the actual beneficiary filled in. This one was also issued in 1702 by the bishop of Cologne, for the German army, and the final one, from that year was produced by the duke of Marlborough, commander of the English army. In this period of more civilised warfare, the people of Oost were lucky. Formerly, to get a sauvegarde, they had to promise not to buy one from the enemy, a real Catch 22 situation. This same duke of Marlborough, by the way, built Blenheim Palace from his savings, one of the biggest estates in England, nowadays a world cultural heritage site. Winston Churchill, the grand-grand-grand-grand-son of the Duke of Marlborough, and equally warlike or perhaps even more, grew up there. He even wrote a bombastic, and not all too historic, biography of his forefather.

This is the most beautiful topographical map I know: the map of the Rhineland made by Colonel Tranchot on orders from Napoleon from 1804 unto 1813 Again the village of Oost, with again something very wrong with the position of our farm, although quite different from the 1832 cadastral map. The French liberation under Napoleon was probably the worst episode in the history of this region, and that includes the Second World War: heavy taxes to pay for the Napoleonic wars, and, worst of all, the conscription of young men to fill the armies of that war. About six percent of the population got conscripted, and three percent died. Which is a lot.

I know the names of five young men from my village who died in Napoleon's armies, respectively in Spain, Eastern Germany, Poland, Austria and Alexandria. Only one died in battle, all others from diseases. Armies are really very dirty places. The young man in Alexandria died from homesickness, according to the bureaucratic report. It's really true! My great-grandfather, who was born in 1869, and died at 94 when I was nine, must have known of these young men, and was even very likely related to them. Can you guess, by the way, who the young fellow with the apple is? It's really not that long ago.

After Napoleon, peace reigned for a century, perhaps the happiest period for centuries. Then, in 1914, Germany attacked France. The Euregio is not only one of the few thoroughfares from France to Holland, it is also one of the few easy east-west thouroughfares from Germany to France. The Germans left Holland untouched, to keep open a trade route through a neutral country, so the complete German army, fat Bertha included, had to wring itself through the corridor between the Dutch boundary and the inaccessable Ardennes mountains. In their hurry they completely bombed down the city of Visé, just to the south of the border. My grandfather, who, for once, was glad to be Dutch, vividly remembered having seen Visé burning from just over the border. And then came the second World War and the Nazi occupation, ending in 200 people getting killed during the bombings of Kerkrade, on the Dutch side of the German border. By allied bombers, by mistake. Perhaps it was an error in the pilot's maps.

So, maps aren't as innocent as they look, and neither are they just things of beauty. They have been the means of the most brutal wars and oppression by the nation states from 1500 until 2000, especially when they were (or are) secret or non-public. It is really not all that far-fetched to see them as the Flowers of Evil, "Les Fleurs du Mal".

You can easily see why Maastricht was considered the best fortified city of Europe, and why its assaults were publicized on a European scale. Nowadays, we do not need those walls any more, and national frontiers are disappearing, certainly in this Euregio, although they remain very tenacious yet in their dead agonies. And perhaps only a beautiful map like the one from Napoleon can help us to become aware of the essential community of this region, and of all the world, even if that is certainly not what it was made for. In this globalized world, the era of the nation states is nearing its end. We do not know what will follow, but things seem to be flowering at places. So, perhaps, now is the time, and here, in this World Wide community of Open Source GIS hackers, is the place, to finally, finally, do something good with these horrible, and yet so beautiful, maps.

Thank you very much for you attention.

Georeferencing Historical Maps over the Internet

The High Seignory of Oost near Maastricht

Foss4GIS 2007

Victoria, Canada, Sept 26, 2007