DavidBethany

Each airline is different. On my recent trip to MN on Airtran, they specifically prohibited GPS while on the return trip, Delta had no specific mention for GPS so it fell in the same category as other electronic devices. I used to ask the pilot, not the crew, when boarding and if they said yes, the crew would not interfere. But that was before... GPS works great on commercial flights if you get a windo seat and it allows you to observe some interesting things like: Speed the plane leaves the ground and lands, about 170 MPH. Actual altitude and know when the pilot starts down. It is a lot sooner than you think. They usually spend a great deal of time in a slow climb, leveling off at some mid-point, then throttle back a little in a slow descent, maintaining the same speed. You may see speeds in flight at close to 600 mph if you get a good tail wind, especially Eastbound. Your own ETA (add about 10 minutes for manuvering to land and you will be pretty close). You will know what cities you are over so you should know the rivers and other features. Also, you can plot the whole trip on your computer when you get home and color code the track line by speed. Load the track on your next flight to the same place and compare. As for altitude. Assuming your GPS reads GPS altitude, and not altitude derived from air pressure, the reading will usually differ from the what the pilot reports by about 300', maybe more. Pilots fly at "Pressure" altitudes, not "true altitude" above 18,000 feet. Above 18,000 feet, the requirement is to set the altimeter pressure reading to a standard median of 29.92" and not the real pressure. That way, all planes are at the same level for any reported altitiude, regardless of how accurate or true. The importance of flying an altitude assigned by air traffic control - called "flight level" as in flight level 330 for 33,000 feet - is to maintain vertical separation from other air traffic. Once safely above any possible contact with the ground, or mountains (cumulus granite) the real altitude, or even height above ground, is not so important. So as commercial aircraft fly from higher pressure areas to lower, the altimeters would show higher altitudes if they really stayed at the same level, but they don't. They all descend to maintain the same pressure altitude as the outside pressure decreases. This means they are not flying the "real" altitude, which can be easily computed from a GPS, or by setting the altimeter to the real-time pressure reading of a ground station below, but that is not important. These rules were in place long before GPS. Flying at speeds around 8 miles a minute, a pilot would need to set and re-set the altimeter about every 5-10 minutes to have the correct reading. So they all fly at the wrong altitude (most of the time) and it works just fine. So when the pilot says you are now level at 33,000, you may see a different reading but it will remain constant for all the time you are level. Unless he is fibbing a little, as they are known to do. More trivia: "cabin pressure" is, by law, no higher than 8,000 feet. In other words, the cabin is pressurized to the same or more pressure than you would experience at 8,000 feet so we can all get oxygen in our blood when we breath. If the cabin pressure is breached, put that mask on quick and expect a wild emergency dive to below 10,000 feet. David

The idea is so save room and make the unit as thin and light as possible. This GPSr is marketed to folks who need to wear it on their wrist like: marathon runners, wind surfers, kayakers, etc. As noted, they have a model that takes batteries also if you are willing to wear something thicker and heavier on your wrist.

Not to be picky or tedious here but I can clarify this issue a little more.. This is where the picky part comes in. Yes, if properly calibrated and, just as important, RECENTLY calibrated. As noted, atmospheric pressure changes over time, sometimes rapidly. I suppose if I was hiking, and started at a known elevation, setting the elevation in the GPS then the smart thing to do would be to note the elevation at camp, and the change the next day. Then, reset the altimiter to the last know good reading from the night before as a starting point that next morning. Pilots set their altimeters to a barometric pressure setting, adjusted for sea level, that is reported constantly on recorded weather broadcasts and airport information frequencies. Above 18,000 feet, all altimeters are requred to be set to a "standard" setting of 29.92" but that is another subject. The alternative is to set the altimeter on the ground to a known ground level elevation. If you were hiking near an airport, had an aviation radio receiver, and knew the frequncy for the ATIS (Air terminal information services) for that airport, you would always have up-to-date pressure readings, adjusted for sea level, to set in your altimeter. Barring that, you could do the same with a weather radio, I suppose, if it were really important to you. This is the best of both worlds. My Meridian Platinum has a barometer, but it is not tied to an altimeter or used for altitude calculations. It is really just a silly feature but I suppose it is usefull for predicting weather. It also has a thermometer but the sensor is inside the unit so it shows the temperature in there as it warms up through use. I don't know what they were thinking but I'm sure the marketing people like it. The magnetic compass is a great feature. But again, I digress. The most accurate GPS elevation readings, excluding WAAS, are when at least one of the sats used for data is low on the horizon. GPS altitude readings are based on a geodatic datum but reported as feet (meters, etc) above sea level, same as an altimeter. David

Bethany got me started with GPS by giving me a GPS-315 as a Christmas gift in 1999. I have used it as my one and only GPS for boating, flying, driving, and hiking until just recently buying a Meridian Platinum unit. The 315 will interface with any good program as it does not use proprietary formats like most of the modern units. Many people have developed their own databases for the 315 and these can be purchased or downloaded for free. The 315 is a great GPS and I see no reason it would not be perfect for Geocaching unless you feel you must have a map. The firmware includes a pretty decent amount of waypoints. The averaging thing was corrected long ago and was not really that big of a deal. As reported, the satellite lock is very strong. The screen resolution is great and, for hiking, this is a really solid little unit. I was going to sell mine but I decided to keep it forever instead. The Datasend software is no longer supported (updated) by Magellan but this is no real loss. You should be able to pick up a used 315 for about $85.00 and pay no more than about $10.00 - $25.00 for any old version of Datasend. Then buy add-on, current databases off of E-Bay for less than $10 to load current info. You can even turn it into an Aviation GPS, what Megellan called their Model 315A or 320 (all the same unit) for free. David

I frequently download waypoints I create to any of the many programs I use that communicate with the GPS. I can then edit the waypoint lists, add or delete points, then save back to the GPS. HD, or SD card. If yoiur waypoints are valuable, you may want to back them up in CD, just as yoiu would back up any other computer data. The Meridian has a 500 WP limit in memory at any one time but I can store as many Waypoint files as an SD card will hold and load the files as needed. I have to keep the files to 500 WP's or less. The practical limit in memory is less when using Magellan's DirectRoute software for street routing as this software creates temporary waypoints on the fly when creating a route and will not create a route if all 500 slots are already filled. If you need a lot of waypoints, and you are comparing this one feature of different GPS models, keep in mind the notes limitation also. For example, I think the Magellan Meridian will only allow 200 waypoints to have notes. It doesn't matter how abreviated or verbose you make the notes. My old GPS-315 will also store 500 user Waypoints but more than 15,000 POI's (down from 20,000) can be imported from their Datasend software and other databases. This unit does have mapping so the internal memory has more room for the small WP and POI files. David

I don't know about Garmin products but I have a GPS-315 and a Meridian Platinum. The SD card not only allows you to load large blocks of data but gives you the flexibility to use a much smaller map set and waypoint database when that is all you need, keeping larger map regions on the card to load at other times. Same with the waypoints. I have lists and sublists for: Geocaching, driving, marine, and aviation. A word on the Platinum. For $20 more than Gold, I bought it. The barometer/thermometer is sort of interesting but otherwise useless. Unless you care about the temperature inside the unit. I have left the unti on in my car to get a barometric history (and see how long the batteries last) but the novelty and thrill of ownership peaked pretty quickly. The compass, however, more than justifies the price, especially for Geocaching. With a true magnetic compass, you can stand in place with an arrow on the compass card pointing right to the coordinates while reading the distance in feet. Only a mag compass will allow you to do this. A GPS compass display is useless once you stop moving. That being said, one caution is the compass uses battery power when in use. For ANY screen on the unit where a customized field is set to "heading" the compass will be activated to read magnetic heading. If the map screen is set to magnetic north, it will also use the compass so I keep it off for all screens except the compass screen. DAvid

The Magellan GPS-315 claims to hold 20,000 POI's or points of interest. You can add 500 waypoints and create up to 20 routes. It will only hold the current track in memory and has no means of storing a track, except to convert a track to a route. The Magellan Meridians allow 500 waypoints also in hard memory at once but you can store many more on SD cards. As many as the card holds. Then, you can load the groups from the card, but the file must not have more than 500 and loading a group from the card adds to any waypooints that are already in memory if you didn't clear memory first. The pratical limit is smaller than 500 since mapping software like Mapsend DirectRoute creates waypoints and routes on the fly and needs this same hard memory to do so. Map regions, tracks, routes, and Waypoint lists can all be stored on SD cards, swapped back and forth to the computer for editing, loaded from the computer to the card using a card reader, or loaded to the card from the GPS.