Coding classes for kids in Chemnitz

import csv

with open("chemnitz_trails.csv", encoding="utf-8-sig") as f:
    for row in csv.DictReader(f):
        signed_km = float(row["LAENGE"])
        map_km = float(row["SHAPE__Length"]) / 1000
        stretch = map_km / signed_km
        print(row["NAME"][:7], signed_km, "km on the sign,", round(map_km, 1), "km on the map, x", round(stretch, 2))

The map layer is stored in the Web Mercator projection, the one most web maps use. To flatten a round Earth onto a square map, it stretches everything more the further you are from the equator. Chemnitz lies at about 50.8 degrees north, and at that latitude the stretch is 1 divided by the cosine of 50.8 degrees, about 1.58: exactly the factor the children found. Our explanation, from the layer's own projection code; the city's signed lengths are the ones to walk by.

import csv

stretches = {}
with open("chemnitz_trails.csv", encoding="utf-8-sig") as f:
    for row in csv.DictReader(f):
        stretches[row["NAME"]] = float(row["SHAPE__Length"]) / 1000 / float(row["LAENGE"])

typical = sorted(stretches.values())[len(stretches) // 2]
odd = min(stretches, key=stretches.get)
print("Trails:", len(stretches), " typical stretch:", round(typical, 2))
print("Odd one out:", odd, round(stretches[odd], 2))

Data: Stadt Chemnitz, Wanderwege, published on the Chemnitz open data portal and listed on GovData through the Freistaat Sachsen, reused under DL-DE BY 2.0 (Namensnennung), file fetched 3 November 2026. Map length is SHAPE__Length in metres, divided by 1,000. Stretch factors and the projection explanation are ours.

Every trail stretches by 1.57 to 1.60, except the Lohse-Uhlig-Steig at 1.39. The program cannot say why. Perhaps the line on the map is drawn more simply than the real path, perhaps the signed length includes a stretch that is not on the map, perhaps one of the numbers is out of date. Children are asked to write down two possible reasons and how they would check each one. Spotting the odd one out is the start of an answer, not the end.

1. Ages 6 to 7, no screen. Measure a path across the room with your feet, then with a parent's feet. Why are the numbers different, and which one is "right"?
2. Ages 8 to 9, Scratch. Make a list of five signed lengths. Multiply each by 1.58 and show the map length. Then put in 13.8 and ask the sprite to say "odd one!" if the map length is not close.
3. Ages 10 to 12, Python. Run Version 1, then change Version 2 to print every trail whose stretch is more than 0.05 away from 1.58.
4. Family challenge. Pick one short trail, walk it, and count your steps. Does your number agree with the sign or with the map?