Henry Grabar is a staff writer for Slate’s Moneybox and a former fellow at CityLab. He lives in New York.
Everyone wins, except your employer.
The British Library needs your help.
It is in the process of turning its gargantuan collection of maps into a digital resource tied to Google Earth. This allows people to see ancient and modern maps at the same time, with a slider to fade between the centuries. It's easy to compare a city to its younger self, or observe the geographic impact of dams, bulkheads, and embankments. You can spot the mistakes -- or marvel at the accuracy -- of mapmakers working in the pre-electric age.
But for this to happen, the Library needs people to set up control points linking old maps to satellite data. It's simple: you find recognizable locations and join them. Then, the software uses your input to mesh the two maps. (You can learn about georeferencing and be assigned a random map here.)
I tried my hand at an 1857 map of Alexandria, Egypt, surveyed by the Lieutenant E. W. Brooker in 1857 for the British Hydrographic Office.
First, I gathered a handful of control points, which requires some keen detective work. The shape of the coastline isn't terribly reliable, and the street grid has changed almost beyond recognition.
Once you're done with that, register (all you need is an email address; it takes seconds), and you can clip and drag your map into Google Earth. Here's a section of the geo-referenced map:
That's a fade between Google Earth's image of the Egyptian coast and Brooker's 1857 map. It's easier to see when you're controlling the slider, but it's pretty damn accurate. There's been a huge amount of landfill, particularly on the southern side of the city. But Brooker nailed the proportions.
How good is this 150-year-old map? The Map Analyst tool will tell you:
That distortion grid can show where the mapmaker made mistakes, or which control points might be off. In this case, both Lieutenant Brooker and I did a pretty good job. Mostly him though: his map has a mean position error of .001414. (I assume that's kilometers, but it doesn't specify.)
So far, the British Library (and its map fans) have geo-referenced almost 1,500 maps. But there are over 700 new ones online, from Somalia to Yugoslavia to the South Pacific, waiting to be geographically fixed.
The BL also keeps some statistics about who is working on the maps. Six users do 75 percent of the mapping.
Mauricien has mapped 4,014 control points; Sue White is in second with 3,300 points. (It's a minimum of five control points per map, so that's a lot of control points.)