While sitting around the house at night, I have been using that map ruler to record the DMS measurements that I have gotten from all of the control points and benchmarks visible on my NAD 27 map that I gridded into nine seperate 2.5' rectangles. Though more fuss to get data down on paper, I feel that it's a more exact method of measurement than the grid roamer just because there's fewer lines, the meridians are more true than the 'square' UTM grid lines and the scale of the ruler measures to a smaller area on the map than the 'crowded' scale of the roamer.
The reason I'm using the USGS markers is because they are all marked on the maps with very fine, tiny 'X' marks that are easy to measure and they are complete with elevations, so I can check the GPS elevation reading against the map, too. I have been meticulous in my map measurements, just to see how close I can get to what the GPS says when I lay it on top of the bronze discs embedded in the tops of the monuments. I am happy to report that my measurements have all been accurate to within at LEAST 1/2 second. To me, that kind of accuracy measured from a map with a simple little plastic ruler is uncanny! Best $5 bill I ever spent to tie a map and GPS together......so far, I have watched the GPS screen as I approached my goal-spot and when it 'zeroed out', I started looking around and I have been able to spot the marker within 5'-10' of where I stood......NO kidding!