Tracking on grass or asphalt can be difficult because there often aren't any good objects to use as markers, so you need to place your own. In this video, Laustinart shows one way to remove markers, but it only works well if the camera isn't moving too much. If the camera is walking through the scene you'll need something else. I came up with the following technique that seems to work pretty well.
For markers, I used golf balls propped up on tees for shooting in the grass, and poker chips on the asphalt. (You can get hollow plastic golf balls at Wallmart pretty cheap. Get white ones. Yellow is too similar to green.) Avoid placing markers near shadows or anything else that might get Copy/Pasted into a wrong spot.
Start by using Laustinart's technique for creating masks around each tracking marker. Don't forget to parent each mask to its marker.
In the node editor, the Movie output splits in two, one going to an Alpha Over, the other going to a Translate node which then leads to the Alpha Over.
Create a Mask node for the mask that you made in the Movie Clip Editor. Give it some blur to make the edges less obvious. Plug the Mask into the Fac socket of the Alpha Over.
Now you'll need to offset the second image a bit using the Translate node. How much and which direction may depend on your scene. If none of the markers leaves the frame you can probably get by with just using the "X" value. Use just enough that you're sampling an area near the marker, but not getting the marker itself.
If markers are disappearing off the bottom of your movie, you'll need to add some negative "Y" displacement.
Here's the node setup for the grass scene...
And here's the asphalt scene. The only difference is the asphalt scene only Translates in the X direction, while the grass scene uses both X and Y.
Here's a video. I don't have a decent video camera. I use my still camera which takes crummy video, and then Youtube makes it look even worse.
I'd love to see what someone can do with an HD video.
This technique should help improve your camera solutions. You can place the markers in an orderly grid which should help when selecting the Origin, X, and Y axes. After you've got your solution, go into the 3D viewport, go to the Top view and see if these markers are in still in a grid. If not, it probably means your Camera Data is off.
If you bring a tape measure and measure the distance between markers, you can use that with the Set Scale feature.