GPS vs AltimeterI've used an altimeter for years and picked up a GPS (Garmin E-Trex Legend) summer. An altimeter will not calculate speed. It is essentially a barometer. If barometric conditions change, so does the altitude readout. The unit needs to be reset at a known altitude on a regular basis.
The GPS altitude readouts were terrible for sometime, but now are very accurate. I'm no expert, and it gets pretty complicated, but the GPS should be reading four satelites before the altitude measurement is accurate. There is a screen on the unit that shows the number satelites that are being read. I used the GPS on several hikes this summer and found the altitude measurements to usually be very accurate. However, on several occasions they were off considerably! The speed and distance measurements, mostly tested in a car, were uncannily accurate. At slow, ie walking speeds, they're often not that accurate
Many Garmin units will connect to a computer. Check and make sure, for $70 I have my doubts. If it's one of the E-trex series for that price it's a buy. I paid $170 for mine (srp arnd 270) with 8 megs of memory. The softwear is extra, about $120. This includes mapping for the entire US on two CD's. The maps that come on the units only show major roads, the topos are downloaded from the softwear. The Garmin unit will download into some other mapping systems, for instance National Geographic, but will only upload from their proprietary softwear.
Find out what unit is being advertized and go to the Garmin website. It will give you all the features.
I've used mine mostly in the summer months and the battery life is quite good. About eight hours with rechargable batteries. If you're not using these they are VERY highly recommended. I wonder how the battery life will be on the slopes? I know it's terrible with my camera.
I have some questions myself about the validity of the speed and mileage calculations in a skiing environment. For instance, on a run with many short turns will it be adjusting fast enough to track each turn? Guess I'll just have to take it out and make some notes and download the run and see.
The map readouts are small. If you know about where you are on a map it's not hard to locate yourself precisely, but often difficult from the GPS units itself.
Keep in mind that they are electronic instruments and can fail (batteries go out at a critical time, etc). Nothing replaces good solid map and compass skills.
I hope this helps.