[OC] LiDAR-derived map of 200-foot trees in the Seattle area Visualization

What This Visualization Shows

This data visualization displays "[OC] LiDAR-derived map of 200-foot trees in the Seattle area" and provides a clear visual representation of the underlying data patterns and trends. The visualization focuses on Source data: Washington DNR LiDAR portal.

It took me many hours of work but I finally finished this map. See the full interactive map here on Tableau: [https://public.tableau.com/app/profile/darius.burt/viz/200-FootTreesintheSeattleArea/Dashboard1](https://public.tableau.com/app/profile/darius.burt/viz/200-FootTreesintheSeattleArea/Dashboard1)

I mapped out essentially every 200 foot tall tree around Puget Sound. I could have expanded more but the density of 200 foot tall trees becomes obscenely high if you stray too far into rural areas such as the Cascade mountains or Olympic National Park. The map already contains over 100 thousand trees, and I had to individually verify every one. At some point I will make another map of the whole state but with a higher height threshold. I was able to map out all of San Juan County, Island County, Thurston County, Kitsap County, the eastern half of Mason County, the western halves of Whatcom, Skagit, Snohomish, and King counties, and a portion of Pierce county.

Although 200 feet may seem incredibly high, Washington is actually home to many of the tallest tree species in the world, including the Douglas-Fir and Sitka Spruce, with many examples growing over 300 feet tall. Other extremely tall trees native to the State include Western Redcedar, Grand Fir, and Western Hemlock among others, all of which can grow well over 200 feet.

If you examine the points on Tableau, you will see a number of different fields for each point. Firstly, there is Terrain Adjusted Height. I made this field to deal with an extremely frequent issue with tree heights on a CHM. Trees that grow on terrain that is rapidly changing can have extremely inflated values. If part of a tree is leaning off the side of a cliff, then measuring straight up and down, which is how the CHM works, will end up subtracting the DTM value at the bottom of the cliff from the top of the tree instead of the base of the tree. There isn't normally an issue if the ground is relatively flat, but sudden enough drops will inflate the height. To combat this I take the DSM value attributed to the top of the tree, and instead of subtracting the DTM value directly below it, I subtract the maximum DTM value for a wide radius, say 50 feet, around the tree. This ensured that the DTM value will not be the bottom of the cliff but at least as high as where the tree is growing from. Generally, the Raw Height, or just the standard DSM - DTM calculation will be fairly accurate, but if the Terrain Adjusted Height is much lower than the Raw Height, then that Raw Height measure is likely inaccurate due to rapidly changing terrain.

There are some other interesting fields I provided as well. I also list the relative height of the tree, which takes the raw height value and divides it by the average height value for a 100 foot radius around the tree. This can show you how much the tree stands out from its surroundings. I find it to be an interesting metric as older growth trees are more likely to be less crammed against other trees. I also find trees that are more prominent just more interesting in general to look at as they seem much more impressive. The LiDAR project the tree comes from is also listed, along with the resolution of that project. Most are at 1.5 feet per pixel, although 2 of them were older projects with a worse resolution of 3 feet per pixel. Finally I list the distance from the tree to the nearest road or trail and the elevation that the tree is growing at. Additionally, clicking on a point will pull up a link you can click which will open a Google Maps webpage marking the point. I also added a filter where you can easily show or hide the extent of the project, displaying which areas I covered when looking for tall trees.

I would also note that height is often not as tied to age of the tree as you might think. Diameter of the trunk is often a much better measure. I have seen many exceedingly tall trees, some upwards of 250 feet, that are just not very thick and don't look very old. A lot of older trees can have the tops fall off in a storm or for whatever reason, meaning many of the oldest trees are actually not the tallest. For example the tallest tree in Seward Park is not particularly thick, but there is one less than 180 feet tall that seems to be the thickest and one of the oldest in the park, marked as a heritage tree and with a diameter of at least 70 inches.

In order to map all the trees I downloaded LiDAR data from [https://lidarportal.dnr.wa.gov/](https://lidarportal.dnr.wa.gov/) which has LiDAR data for the whole state available to download. For multiple areas I downloaded the DTM and DSM files. DTM stands for Digital Terrain Model, which represents the bare earth, while DSM stands for Digital Surface Model, which shows the elevations of all objects. By subtracting the DTM from the DSM data we can generate a CHM, or Canopy Height Map. Now you can look through this map manually and identify tall trees and mark them with points in a software like QGIS and manually enter all the heights, but there is a much more efficient approach. By writing a script in a language like R or Python, you can automatically look through the map and mark all local maxima within some certain window size above some certain height threshold. Basically marking say all points that are the highest point within a 15 foot radius and are over 200 feet. This is very good at marking the tops of trees, although inevitably other objects will be marked as well, and trees that are irregularly shaped can end up with multiple points marking them. This is why manual review is required for all the points to ensure they only mark real trees, and each tree only has one point. Although I spent quite a while looking through the points I wouldn't be surprised if a few non-trees could have slipped through or some trees were marked more than once. Maybe some advanced automation technique could be used to validate all the points but that is beyond my capabilities.

I hope you enjoy my map and can visit some of these trees for yourself., which allows us to understand complex relationships and insights within the data through visual storytelling.

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