While practicing with my Etrex and the new settings Lyle gave me I tried something and here's the results.
1. With the sat/nav set to Nad 27 conus I took a waypoint of my home in the driveway:
RESULTS = e0601547
n4725445
2. With the sat/nav set to Nad 83 (datum on the USGS map) taken in the same spot in my driveway:
RESULTS = e0601577
n4725670
3. Using a USGS map and a corner Roamer scale I read = e0601560
n4725647
My Conclusions:
Follow the advice that when using a sat/nav set it to the datum on the map because you could be 25% off as in this case, and 25% of 1,ooo sq. meters is as you know 250 meters which is a good amount in say a wooded area or passing a person laying down unconsious.
My math could be off a bit, but you guys get my point! I love this stuff!
Barry
Good for you for experimenting. Very good for you. I have two additional thoughts.
First, IMO there are two kinds of "knowing" — intellectual and emotional. In a previous life, I sometimes taught how to diagnose some things. My students dutifully memorized what I told them to and answered test questions correctly. Yawn.
But there came a time when they actually had to commit themselves to what they learned in a real-life environment, and when they did, and they found what they'd learned actually worked, they were impacted emotionally in a way which cemented their knowledge. (I know because up to three years after I'd taught them, some would return to tell me, with wide eyes and surpassing enthusiasm: "Remember what you taught us about ...? Well, it works!")
Having compared coordinates for the same location using different datums, I suspect you'll never forget the importance of using the correct datum. And that's a big deal, at least in these parts.
Second, in teaching GPS/SATNAV to our SAR members, we strongly emphasize the importance of using the correct datum. We have a powerpoint slide of one location correctly placed on a map with the proper datum, but incorrectly placed using 5 - 6 incorrect datums.
Another slide shows what happens if you go to the right coordinate numbers, but you are using the wrong datum: you're on the wrong side of a river in a deep valley with no way to communicate with the outside world and with the only river crossing 2 miles away along a gnarly trail.
In fact, every time we have a discrepancy in location (my receiver doesn't agree with your receiver) the first thing we check is whether or not we are using the same datum.
FWIW. We (our county and the other 8 CORSAR counties) used to use NAD27, because it was the datum used to create all our 1:24,000 USGS paper topo maps.
But we changed a year or two ago to WGS84 because we no longer use topos printed by the USGS: we now print our own maps using National Geographic TOPO! or TNP, and can select which datum we wish to use. (Air and Marine assets use WGS84, and it's just easier for us to do so too.)