Mapping Power - Power Analysis in TopoFusion Pro


Mapping power output is an exciting new feature of TopoFusion Pro. TopoFusion can load power enabled .TCX files (for example, from a Garmin Edge 705), or you can use existing GPS files and combine them with PowerTap .CSV exports. If you have TCX files with power you can skip to things to do with power.

Attaching PowerTap data to GPS points

It's easy to sync PowerTap with GPS using the "Auto Offset" function in TopoFusion. What you need:
  • A .CSV export from WKO or PowerAgent
  • A .GPX or .TCX file from the same ride
For an example file, see here: example_power.csv

Load the GPX/TCX file in TopoFusion, then go to Power Analysis (found under the Analysis menu). Make sure the GPX/TCX file is selected in the "GPS File" dropdown. Now use the "..." button to find the associated .CSV file. Note: the .CSV export can be of any recording rate and from power meters other than PowerTap (WKO can export any file to PowerTap .CSV format).


Once the CSV is selected, hit the "Auto Offset" button. TopoFusion will search for the offset between the powertap and GPS file. If you started your GPS and PowerTap at the same time the resulting offset should be small (only a few seconds). TopoFusion will search 30 minutes in both directions, but if your files need a larger offset than this, you will need to seed the offset with an appropriate value. Once you have run the Auto function once, or simply hit the "GO" button, you will be given the elapsed time for both the GPS and PowerTap files. This is a reference in case you need to manually set a larger offset.

The offset is applied to the PowerTap time values. For example, an offset of negative 30 seconds means that the PowerTap started collecting data 30 second before the GPS. If you forgot to turn the PowerTap on while the GPS was collecting points you would need a positive offset. Remember that the Auto search function will look a half hour before and a half hour after the current offset value.

How does the Auto Offset function work?

The Auto function works by looking at the errors between speeds in the GPS file and speeds reported by the powertap. An estimate of this error is provided for the given offset in km/h.

Saving power data

Note that GPX files cannot store power (or heart rate / cadence) data. So once you attach power data you should do "save as" on your GPX file and make sure it is saved to TCX format (readable by Garmin Training Center). This way you'll only need to sync your files once. A warning message will appear if you try to save a file to GPX that has power data.

Interpolating GPS data

Sometimes GPS files (particularly long rides collected with an Edge unit) don't have a recording rate of one point per second. Yet the power tap collects power readings at near second resolution. In order to get more resolution you may want to create more GPS points before attaching PowerTap data.

First make sure the correct file is selected in "GPS File" dropdown. Then hit the Interpolate button. You will now have one GPS point per second. It's recommended that you save this file out (as a new file), then proceed with attaching power data to it.

OK, I've got power data with GPS points, now what?

Here's where it gets cool. First thing to do is go to the Map Menu. At the bottom of the menu there are six options for Track Shading. Choose power. Your GPS track will now be shaded according to power output.

Power profiles


Now, right click on the GPS track and bring up the profile window. Select Power in the dropdown for Y Axis. As you move the mouse over the power graph the current position is updated on the map, and the current power reading (as well as many other stats) are updated in the profile window.

Power intervals


Now, let's look at pieces of the ride. Go to the Analysis Menu and choose Lap Analysis. By default the ride is broken up into 1 mile intervals. You can change the interval length, but note the stats -- average and maximum power are computed for each piece of the ride. Selecting an interval (entire row) highlights that piece of the ride in the map. Double clicking on the interval centers the map on the segment and highlights it.

Note that you can sort the intervals by any of the available columns. For example, you can easily find which quarter mile of the ride had the highest average power. Just sort by average power and double click on the top row (as the above screen shot shows).

Finding specific points


Right click on your TCX file in the Active File List (main window) and choose "File Properties". The File Properties dialog has a tag that contains all track points. You'll first need to fill the track point table by hitting the "show points" button. You can then pinpoint specific power values. In the above example all points have been sorted by power and the highest value for the ride is shown. Double clicking on a point highlights (green dot) and centers the map around the point. You can also delete specific points from this dialog (by right clicking on them).

Look for more power functionality in future versions of TopoFusion!