That stick is no ordinary ruler. It's called a Biltmore stick, and has a scale that allows the user to sight the outside limits of a tree's diameter from a single point of view.

On Thursday near 7th and Madison streets, The Chronicle noticed a guy wearing a bright yellow vest with electronic gear and some sort of measuring stick. We had a pretty good idea what it was about, having recently reported on city council’s approval of a $243,500 contract with Davey Resource Group for a GIS-based inventory of trees in the public right-of-way as well as in parks.

Marcia Higgins, one of two councilmembers for Ward 4, had cast the lone vote against the contract, and had explained at Sunday night caucus two weeks later that she would prefer to see the money for the project, which is coming out of the storm water fund, spent directly on storm water.  She also wondered if the work could be completed more cost-effectively as a Boy Scout service project.

It’s not Boy Scouts who are doing the work, but rather four guys from Davey Resource Group.  One of them is Wes, the guy in the yellow vest, who chatted with us as he took down a couple of trees’ vital statistics: height, trunk diameter, type (genus and species), condition, and location.

Early in the conversation, we asked him what kind of tree he was recording at the moment, and he said he figured it was a sugar maple, “but it looks a little different from the sugar maples I’m used to seeing back east.” In this case, “back east” means New Hampshire, where he lives.

Wes uses a clinometer to get a bead on the height of the trees. The device measures angles, and the rest is the magic of geometry.

He was flown in on Tuesday, just two days earlier, and he figures the four-man crew, which is working Monday-Saturday from around 7:30 a.m. to 5:30 p.m., will need a couple months to complete the inventory of around 60,000 trees. The pace of around 260 trees a day that Wes reported would take around 60 working days.

Not all of the four tree guys who are working on the project came from outside the region.  For example, the project manager drove up from Ft. Wayne, Indiana, which means the crew has a vehicle to drive people from their hotel out by Briarwood Mall to their job site for the day. On Thursday Wes was working 7th Street from north to south and needed to get to Pauline.

In Wes’ tool kit, along with the GPS unit (for precise location information) and tablet-style computer, were two old-school analog devices: a Biltmore stick for estimating tree diameter and a clinometer for measuring height. Through the power of applied geometry, they’re not just guessing those stats. Wes wasn’t just guessing about that sugar maple, either. He’s an ISA-certified arborist, and has been working in the field for around 10 years.

Why is the city of Ann Arbor conducting a tree inventory, and why is it being paid for out of the storm water fund? The 1994 inventory was based on street addresses, and a pilot attempt to geocode that inventory to make it compatible with the city’s GIS system proved not to be cost-effective. In addition, the arrival of the emerald ash borer in 2002 meant that city staff focused on removal of dead and dying ash trees, which diverted resources from the ongoing update of the inventory. The 1994 inventory did not include park trees like the current project does.

As for the rationale for using storm water monies to pay for the tree inventory, there’s a connection between trees and storm water runoff. A mature deciduous tree uses up an average of 625 gallons of water per year, preventing that amount of water from  entering the storm water system.

So over the next couple of months, if readers spot guys in yellow vests and GPS units sticking out of their backpacks who are taking a really close look at a tree, one of them is Wes. Here’s a question that Wes, the tree guy from New Hampshire, asked The Chronicle: “What’s a can’t-miss thing to see in Ann Arbor?” Readers might want to convey their own answers directly to Wes, or perhaps in a comment below.