I have been working on a personal project to map the little trails on my land, and learned something in the process which might be of interest. Many of you have read the tutorial here, specifically the comments on page 23 about map resolution: http://cgpsmapper.com/download/GM8DocV2.pdf
‘24’ refers to the precision of the latitude and longitude coordinates provided for the object. When declaring the location of objects in the world, the highest precision available uses 24 bits of resolution, which resolves to an accuracy of about 2.5 meters. Consumer GPS units have a best-case accuracy of about 8 meters.
In another thread here, somebody questioned the math that lead to the 2.5 meter figure. I am not sure how the calculation was performed, but the following screenshot will graphically demonstrate it. Using GlobalMapper, 1 foot per pixel aerial imagery, and track points accumlated from many walks on my little trails, I created my map. The points on the polylines are quite close together in some cases - one or two feet apart. I exported the map as a .mp file and then opened it in GlobalMapper again, overlaid on the original. In the image below, the pink lines are the original and the blue lines are the .mp file.
So you can see that the actual coordinates have been rounded off to 24 bit numbers, creating stair steps. Right away it became obvious that I wouldn't be able to make the kind of map I wanted using Garmin's vector format. Now granted, this detail is beyond the accuracy I can expect from my Oregon anyway... but I still wanted to make my map!
The solution was simple; I created a "custom map" (raster image) as a .kmz file. I had overlaid my vector data on 1 foot per pixel aerial imagery, so I just exported everything as a .kmz file in the proper format for the Oregon. But the results still weren't quite what I had hoped for. At close zoom settings, the lines looked too jaggy. So I re-sampled everything at 4 inches per pixel and exported again. This produced a very nice looking map with smooth lines even at extreme zoom levels.
You may not ever want to create a map at this resolution, but if you do then it's nice to know that the 2.5m/24 bit limit does not apply to "custom maps". Of course, you will need an Oregon, Dakota, Colorado, GPSMap 78 or 62 to use this kind of map.