HardLoper wrote:
justcurious wrote:care to elaborate?
Tangents do not add any noticeable distance. If your GPS reading is long then it's GPS error.
http://imgur.com/sXiN2oWThey should make the GPS watches know that you're running the Chicago Marathon, and base the distance off of known checkpoints. If your location is at the 15 mile mark of the race it should assume you have been running for 15 miles. This would be more accurate than the current method.
While I would agree that GPS error is most likely the largest factor in why someone records a race distance longer than the measured race distance, I think you are understating how much not running the tangents adds. Where I live, our city blocks are about 350' east-west and 380' north-south. I have no idea how that measures up against other cities so maybe your mileage varies. If you run around a block on the inside edge the whole way, you run 1460'. Our streets average about 50' wide. Run around that same block in the middle of the street the whole way, you just ran 1560' Run around that same block on the outside corner, you just ran 1660'. That's almost a 14% difference.
I know nobody runs the outside of every corner, and I know courses are usually more meandering than an around the block measurement. But I know there are tons of places that runners add distance. For example, let's say you take a left onto a straight road for 1 mile, then at the end of that road you take a right. Racing the shortest course would be to run the tangent between those corners - a very slow move from one turn to the next. Let's say the street is 50' wide, and there is a water stop 100' from the end, on the left side of that street. If you get close to the water stop, running a good tangent, and then decide you want a cup, to get over to the water and back on course adds close to 100'. Or if you recognize that water stop is going to be on your left when you turn onto the street you stay on the left but then you're not running the tangent, so you've added around 50' (only the distance to get back on course, not the "go to the water then come back" total).
The marathon I work for has ten 90° turns the first 3.5 miles. With average street widths of 50' through that section, even if you just run through the middle of those turns instead of the tangents you add around 250'. I could easily see somebody adding .1-.2 before you even start taking into account weaving around other runners or going wide to get to a water table.
By the way, I love your idea of having a GPS watch with race courses already in them, and having the brains of the watch check your known location against the coordinates of the course map. That would be a big step forward!