In the last year or so I’ve taken an interest in mapping trees. Urban trees, to be more precise; I’d probably not attempt to map a forest that way. I try to record the location, type, and size of each tree. With “type” I mean the species, and “size” I express in height, crown diameter, and circumference of the trunk.

Trees are in many ways an antipole to today’s turbo-charged life. I used to frown when people made a big deal about trees (“OMG they are planning to cut X number of trees for this new railway station”), and I thought what’s the problem, you can just plant some elsewhere. But walking the city with open eyes, I saw many trees that have been where they are for longer than I live, and many that will still be there long after I’m gone. Planting trees is not something for a startup where the venture capitalists demand quick returns. If you plant a tree today, it is the next generation that will be enjoying its shade. And that shade can really make the difference between a livable city and one you’d rather just cross in your air-conditioned automobile.

Why?

In practical terms, if OSM had good data about which streets are lined by really big trees with a dense crown, you could run simulations about city climate with OSM data; you could determine the best routes to walk in the summer, or which pub to go to if you want to enjoy a beer in the shade. (Perhaps I should add an explicit survey date so you can algorithmically extrapolate the tree size.) But going further, if information about trees becomes readily available, people will perhaps appreciate trees more, and think twice before cutting them down to make room for a shopping mall.

But mapping trees also brings me back to the beginnings of OSM. I know nothing about trees.

Just like I knew nothing about surveying when I started contributing to OSM. I wasn’t a trained surveyor then and I’m not a trained botanist now. I bought books and use I apps to help me, and meanwhile I know my Acer pseudoplatanus from my Acer platanoides (though, Tilia platyphyllos vs Tilia cordata still beats me occasionally). Still, I’m light-years away from an expert who can identify a tree without any leaves on by simply looking how the branches are structured. But I’m mapping trees nonetheless. Take that, botanists ;)

Mapping trees requires being on location and surveying. Hence, I am extremely unlikely to encounter competition from the likes of Apple, Amazon, or Facebook who are all using hired labour or artificial intelligence to distil streets or buildings from aerial imagery and stuff that into OSM, leading to many a hobby mapper turning their back on OSM. If you had super detailed aerial imagery then you might be able to recognize the species, and you could guess the height from the size of the shadow… but why would big capital pay people to map trees, there’s no money to be made from knowing about trees.

How?

When mapping trees, I use a standard measuring tape to determine the circumference, and I’ve bought a simple laser range finder for golfers that lets me measure the height of the tree by first aiming at the base of the tree from a distance, then at the top of the crown. It’s a €100 device, not super precise but good enough.

As for the crown diameter, I make do with estimating that. To determine the species, I use an old fashioned book as well as a couple of apps (e.g. Pl@ntNet, Flora Incognita - both free but sadly not open source) that let me take photos and then suggest what kind of tree it might be, though if there’s one thing I can say, it is never trust the app, only use it as a starting point. It can be quite difficult but if you map trees in one particular city you will soon find that the city’s gardening department tends to have their favourites that you meet everywhere. After a while you learn to cherish the exceptions, or marvel an an exceptionally large example of an otherwise rather boring species.

I’ve made myself a simple Vespucci preset that shows only trees and nothing else, and can give them a pink “validation” border if they’re missing one of the properties I deem important. I’ve also made a simple CartoCSS style that draws green tree symbols when a tree is fully mapped, and red symbols in a couple of different variations that let me see which properties are still missing, and I have used that to print a kind of “walking papers” before going somewhere.

Mapping the precise location of a tree with GPS is difficult because the signal gets worse the more leaves you have above you! And more often than not the available aerial imagery has been taken while the trees had leaves and if they’re standing close together, it is practically impossible to pinpoint individual trees on the image. A technique that often works in the city is placing trees in relation to houses: “Ok, this tree is on the right hand side of the road just where house #13 ends and house #15 begins”.

Progress is slow, mostly because to do it right you need to view the tree from up close and from a distance - and because of the research you have to do on account of not being a botanist. But it’s a rewarding pastime. I can recommend it.