Many responses here since my last one!
My probe definitely is not going to try to compete with the high-end ones, but I think it has enough accuracy for the everyday home mill. One test I am doing is running the "Center of Hole" routine that is built in to the latest Mach3 release. I just run the routine, reset x and y using the "Ref All Home" button and then run the routine again to check repeatability. After about the third time, I get errors no larger than .0002, and more often it is .0001 or even .0000 on the Mach DRO's. The first one or two repeats of the test usually shows a greater error than subsequent tests, so that indicates that the first test or two tend to have a greater error, becasuse the error is reduced every time by running the test three times. After three times I never see a difference of more than .0002. Yes, that's 10-thousandths. But not microns!
For those asking about Global Mapper, you can download a trial version that has crippled saves and exports, but it is good for evaluating the program. It is all mapping related, but it can manipulate elevation models in many ways and in reality we are talking about a kind of mapping anyway. When I open a point cloud file it asks which type of geographic projection you want to use. These are standard projection types that are used on maps and they usually take into account things like the curvature of the Earth (which is not enough to worry about in small areas) and how to display a curved surface on a flat one. You can even specify projections for some of the other planets! Depending on the projection system you pick, the point cloud file is opened using (usually) meters as the basic unit of measurement. If your original point cloud is in inches or millimeters it really doesn't matter because we are dealing with units and not inches or meters or whatever.
In the registered version, you can export to a huge variety of file formats for many programs including dxf points or converted to faces.
The trial version can be downloaded at
http://www.globalmapper.com/ (click on "Download Trial" from that page). It can also be downloaded from the USGS at
http://mcmcweb.er.usgs.gov/drc/dlgv32pro/. This version (they call it "DLVG32Pro" may be less crippled than the "trial" version -- I am not sure about that because I have the full version already and haven't bothered with the crippled versions. The large njumber of file formats that can be read by Global Mapper are listed at
http://www.globalmapper.com/product/formats.htm. The point cloud files generated by Mach's wizard is a simple text file that is recognized by the Global Mapper program as an ASCII elevation grid file.
I haven't tried to use Global Mapper for generating a file to convert to G-code yet and haven't given it too much thought. I have so far used it for viewing point clouds with various shading techniques. On road trips I have a laptop computer with a GPS unit in the car with all kinds of mapping software including Global Mapper. I have digital elevation model files (DEMS) for the entire state of Washington (where I usually confine my road trip to). I also have geo-referenced topographic map files and other map format files and aerial photography for the entire state. These can be overlayed in Global Mapper to produce 3D images of the terrain (what it looked like a few years ago when the photos were taken. I can generate a picture of what it is going to look like around the next bend before I go there. Kinda fun. Anyway, Global Mapper is quite a program and could be useful for converting some formats into something useful for machining purposes, especially when dealing with images that you might want to engrave. I really haven't explored this possibility at all.