Lidar-based GIS map of New Hampshire stone walls | Hacker News Reader

Lidar-based GIS map of New Hampshire stone walls

nhgranit.maps.arcgis.com

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For folks not from New England: it's very normal to walk through inhabited the woods in New England and come upon a seemingly totally random stone wall in the middle of nowhere.

Much of New England is 2nd(?) growth forest -- the original forests were chopped down to make space for farmland. The soil is incredibly rocky, and so farmers would go through there fields and chuck the rocks to the side, making the walls. Eventually people realized that New England's rocky soil was not very good for farming/local farming became less important as food was able to be transported longer distances, and much of the farm land was abandoned and eventually reforested -- with the only the rock walls remaining (or at least that's what I was taught growing up there).

Apologies for the vacuous pos, but all these walls out in nowhere has been such a long long mystery to me!

It seemed so hard to imagine why anyone would have needed these, that folks were so worried about property lines! Never any evidence of fence posting, often not high enough to really do much (that might have been a shift over time though, wall falling/dirt gathering). But wall after wall, through forest after forest!

Tom Wessels has a great chapter in Part 1 of his "Reading the Forested Landscape" video series about New England stonewalls. [0] A common myth is the walls were built over time due to the rocks being pushed up by the frost but that's not true!

The over 125k miles of stonewalls were built in just thirty years because of sheep.

[0] https://www.youtube.com/watch?v=zcLQz-oR6sw&t=129s

Stone walls were built because of deforestation caused by clearcutting land to make sheep pastures, made wood for fences unavailable.

Hmm I had always thought that the deforestation was caused by demand for wood for heating and cooking.

Something about this sounds incomplete. A farmer isn't going to waste his time making a wall, especially the dodgy disorganized walls that are common in the region. A farmer doesn't need a shallow wall, stone or wood. The walls you come across in new england in the forest look exactly like people expect, a place on the edge of your farm to dump rocks. Now, maybe not frost grown, but New England has lots of rocks everywhere.

Wessels does indeed say the stone for fences most likely came from stone dumps in _cultivated_ fields that were clear cut for crop fields and, later, the sheep craze and those rocks were pushed up from the ground in those cultivated fields over the winter.

I believe a lot of wood went to ship building too. Though that may be more true of forests near rivers so they could be floated to Portsmouth.

Newburyport was the main one. Many of the Clippers came from there. You could float lumber down the Merrimack after a canal was built at Lowell to bypass the falls there.

That makes a nice "just so" story in the mind of HN and perhaps you'll pull the wool (pun intended) over some of the most ignorant here but that's not how it happened and anyone who has a lawn anywhere freeze-thaw is a thing should know it.

If you clear cut a forest it will be grass or brush or whatever in short order, a year or two. The roots of that vegetation, even just light grass is enough, will bind the soil all together as it freezes and keep the rocks from getting pushed up. You need a mostly dirt field to push up rocks. This means actively utilized pasture or farmland.

The walls were not dodgy and disorganized initially. Most of them were constructed before 1840 and have had close to 200yr of the ground moving under them to break them down. You occasionally see "good ones" on select areas of land where the soil and drainage situation made water in the earth basically nonexistent so therefore expansion and contraction were minimal and the wall aged gracefully. Damn near all of New England was clear cut pasture or farm at that time due to some circumstances in global commodities markets making it very worth everyone's while to raise sheep for wool on any land that they didn't need use some other way.

Nobody would have moved the rocks if they were not in the way, as would have been the case if they simply wanted the wood.

Animals have a habit of trying to get at the bottom layers of fence as they try and reach through for grass on the outside of the pasture and it's really hard on fencing over time. If you are rotating fields between pasture and crops or even just pasture it behooves you to drag the rocks away, straight to the edge is the shortest path. And if you're gonna do that it takes little extra effort to stack them well and create a rock wall for the lower portion of your fence. Spending that extra effort will pay you back in reduced wood fence maintenance.

If anybody is in Connecticut like me, here's a LiDAR map you can use for the state to find your own stone walls here:

https://experience.arcgis.com/experience/4c801e35f200493ebff...

("Hillshade 2023" and "Hillshade 2023 SE illumination" are the two I use.)

I used to work for a company that did GIS mapping of guard rails, lines, and road signs. Part of what we always ended up doing was massaging the data we received from the Lidar mapping to line things up to exact locations for clients. If this map is accurate, I salute whoever spent the time fixing the details.

I don't have much experience with lidar, but I do with machine learning based approaches with photographic data. There has been a massive explosion in the last 5-10 years in filtering and processing algorithm advances that can more readily clean up false positives and insignificant noise. I would assume the barriers to high quality data are quite a bit lower now.

I can see portions of a few walls that are out in spots that used to have a road over 100 years ago but now are reclaimed by the forest. Some of areas are densely overgrown now. Nice work.

I have deep knowledge of only a tiny few square miles of sparsely-populated mostly-forest in New Hampshire.

I see both surprsing accuracy, and the occasional baffling "I wonder what looked dense there??" errors.

So as usual, LIDAR returns non-intuitive results sometimes, and is ideally refined by ground research, when the budget allows.

But I'll definitely check out the apparent errors in more detail next time I'm there. :)

I look forward to finding some of these near me too.

This is neat. I see a few incorrect street names where I live, but also some old stone walls near where I frequently walk.

I'm across the river from you, and my understanding is that a lot of the incorrect street names on online maps come from data sources with older names. E911 apparently caused a certain amount of upheaval with road names.

Some of the online mapping services seem to have compromised by just listing all possible names for a road, whereas OnX seems to be working off of a different (older?) data set than nearly everyone.

This is neat. I'm curious to know what the practical uses of this information are? Anyone know?

My first thought is for historical research. It would be quite a bit of help if you have some old town or settlement map that you can compare to/overlay with a LiDAR stone wall map.

I am not sure, but it may also serve historical preservation purposes if that is an issue, e.g., if administrators are deciding on land partitioning and/or development plans.

That's what I was thinking too. The historical aspect is useful. And I would imagine the position of the walls could be correlated with past land ownership boundaries.

It could be used to enhance topo maps for hiking. If you’re lost and come across a stone wall, you now know where it possibly leads.

I'm not affiliated with the site I submitted, but some just really love stone walls, especially those of us here in New England. I'm part of a "New English Stone Walls" Facebook group that has ~65,000 members.

To me (I'm in CT) there's something really cool to be in a forest surrounded by trees but see a perfectly made stone wall just there in the "middle of nowhere." I think about how much time and effort it took back in the early ~1800s to clear all that land, move all those rocks across fields without modern machinery, and put so much effort into constructing these walls. Some are over 6 feet wide and many are in incredible shape for being put together ~200 years ago.

There's also the "Stone Wall Initiative" spearheaded by Robert Thorson of the University of Connecticut that also has tons of info:

https://stonewall.uconn.edu/

(He also has a really good "Stone by Stone" book available on Amazon.)

It's a testament to what we can accomplish when we don't have distraction machines at our beck and call.

Oh, wow. I had no idea there was so much of a community around this phenomenon. Thanks for sharing.

very cool project and map. I've recently picked up stone masonry so this is super great to view

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