I’ve noticed tiny metal plates embedded in the sidewalk on certain streets

Dear Urban Decoder: I’ve noticed tiny metal plates embedded in the sidewalk on certain streets. What are they for?—Jackie Middleton, DEER PARK

Sidewalks can be surprisingly informative. Every few squares, there’s a stamp with a name and date, indicating who laid it and when. It may look like a promotional tool for construction companies, buy it’s actually about quality control. The city’s streets are one giant patchwork of repairs and replacements, built by any number of private contractors. The stamps let Toronto keep tabs on whose handiwork is standing the test of time (sidewalks can last more than 50 years) and whose crumbled after their first spring thaw. But the most useful marks in the sidewalk are the circular brass plates stamped with triangles. Essentially a low-tech global positioning system, they’re known as “horizontal control monuments.” Toronto has 20,000 of them—placed about every 550 metres—inscribed with coordinates telling surveyors precisely where they are. If you see a surveying crew looking through one of their scopes, odds are they’re standing on one of these circles, measuring property lines and demarcating new streets. The brass plates even have an advantage over GPS: unlike the satellite-based system, they work just fine in the concrete canyons of downtown.