Thanks to some pretty cool software called SportTracks, I have discovered a very interesting phenomenon occurring with my Garmin.
One of the things that people love about the Garmin 205/305 is that it has a more powerful signal antennae than its predecessor, the 201. This is evident when running in wooded areas and around tall buildings, e.g downtown Houston. It doesn't lose signal. However, I found out today that even though the signal stays strong downtown, it's totally whacked out, looking like you're zig-zagging all over the place. And it greatly affects your mileage. Here is a screen capture of the SportTracks software, showing a short portion of my route from this morning along Rusk.
Each red dot represents a single data point recorded by my 305. As you can see, the dot's and calculated route are all over the place when I am between tall buildings. See, the signal is being received strong enough but because it's bouncing all over the buildings, it really confuses the Garmin and you see the result. My total miles for this run were 12.36 but that is from me running all over the place, into buildings, zig-zagging all over the place.
Now, thank to the edit feature in SportTrack, I can move these dots to be in line with the acual route I ran. Here's what the same portion of Rusk looks like after I painstakingly lined all those points up along my actual route.
With the erroneous zigzagging through downtown, the Garmin gave 6.13 miles from the Tennis Center to the corner of GRB. Everyone knows that it's exactly 6 miles. After moving all the points in line with my acutal route, the point at the corner of the GRB was exactly 6.0 miles. I had an extra .13 miles incorrectly added to my run. After I fixed all of the route, the total mileage for the run went from 12.36 miles to a more accurate 12.18, a difference of .18 miles.
What's funny is that my lap pace from where I entered downtown to the GRB was about 11:15 minutes per mile. I remember looking at that and thinking, I'm not going that much faster. What's going on? Well, you see, the time is constant. I mean the clock just ticks. So, if I'm spending the same amount of time zig-zagging all over the place, it looks like I'm going an extra .13 miles, for example, in the same amount of time. So, my minutes/mile is going to be falsely lower. After my corrections, it made more sense and I actually finished mile 6 in 13:08.
Just keep this in mind Garmineers. It's great to not lose signal, but be aware the signal may not be correct when running about tall buildings.