Comparative Photography and Speed Cameras
Comparative photography includes all those techniques where a change between one photo and another is used for detection purposes. It can be military, civil, enforcing planning laws, or could be used for search and rescue. As well as the positive it has unfortunately allowed those less honourable to see it as a form of revenue generation and resulted in the explosion of speed cameras.
We have another article Then and Now Photography that looks at the recreational interest of finding places old photographs were taken and reproducing something similar toady.
People interested in enforcing planning rules, military and some others wish to be able to see what has changed since a previous point in time, and 'then and now' photography allows them to do this.
The historic technique to do this was with negatives, take the first as a negative and then convert the second, taken from the same position, and with the same settings to a positive, like a slide, and sandwich them together. If nothing has changed it will be equally grey throughout. This is because the two, one reversed against the other will mean that the same level of density is throughout. In practice a few outlines won't quite match up, its very difficult to be in exactly the same place and with inexactly the same settings and often there will be a light image as the two exposures are often slightly different. Where something is different the images won't cancel each other out and the position is highlighted. This position within the two photos the 'then and now' can then be looked at in detail to determine the change that has taken place.
As every change will show up, and the 'then and now' need not be taken at drastically different dates, overlying and photographing on one day and then a day or two later can identify even minor changes and identify items that otherwise might be hidden, obviously of great use from a military intelligence viewpoint but it can also be used in searches. So for example if you had a series of photos of a large piece of moorland or mountain and was looking for someone missing, just overlying and comparing the photos may well show up where they are, very quickly.
Rather than with negatives, you can now do this digitally, and with the ability of software to stretch and change perspective within the image its not as necessary as before to have the same position exactly. With large screens you can magnify in on differences and make layers appear or disappear it is now far simpler than it was historically. I don't have experience of military applications in recent years, but one can imagine that with software they presumably have, they would be able to manipulate the contrasts and have pattern matching that would draw outlines on partly concealed items. This added to other techniques like detecting heat sources would make it virtually impossible for anyone to hide anything.
Detecting and Proving Speed - speed, safety or revenue cameras
Detecting the amount of movement in a known time allows you to show the speed that an item is moving, this is the principle of the dreaded road speed cameras, often called safety cameras, but positioned to create revenue from unsuspecting motorists. The most profitable of these is said to be a camera on the A610 in Nottinghamshire that over a 5 year period caught 76,000 people and extracted £4.2million in revenue. Some sections of motorway maintenance has been more than paid for by the revenue the speed limit and averaging speed cameras have produced. No wonder the cones are left on the roads for so long.
The initial detection is carried out by either a beam bouncing back or a pair of cables under the road, to decide who the victims should be. Having detected the victim is over the specified speed for the road, as they pass over a section of road with marks on, two photos are taken at a set time apart, this can be the familiar box on a stalk that takes two in quick succession and has clear road marks or it can be two placed further apart and then can present average speed.
As many people now have speed camera warnings or detection built into their sat navs or as separate devices they are likely to slow down very quickly, so its surprising these hazards don't create far more accidents. As those creating the statistics are the people benefiting from the revenue produced we have no real way of telling what effect they have, luckily most are placed on wide open safe roads, so the change of speed is no more dangerous than the slightly faster speed drivers may naturally go. Luckily these hazards have never, so far, caught me, but I do find looking out for them takes my attention away from the road.
Some places are now getting rid of speed cameras with no rise in the numbers of accidents, and more are looking to do so, and similarly there are plans to close down all the clampers, getting rid of these dishonest means of raising revenue.