GPS co-ordinates

Discussion of the iOS App Dive Log
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awswimmer
Posts: 7
Joined: Mon Sep 06, 2010 10:04 am

GPS co-ordinates

Post by awswimmer » Mon Oct 11, 2010 4:06 am

Really love this app, but am having trouble gleaning GPS data for dive sites - how do I get the GPS location iinto a dive, I am adding the dive at the time just before I go down ;)

support
Posts: 698
Joined: Wed Mar 31, 2010 4:35 pm

Re: GPS co-ordinates

Post by support » Mon Oct 11, 2010 8:13 pm

Hi,

The GPS data is actually associated with the Dive Site. So in order to add GPS data to a dive you need to add a dive site (either directly from the Dive or via the Location tab) and then tap the "GPS Coordinate" row. Once you have the GPS entry screen up you can either enter the location using the iPhone's built in locations services ("Use GPS"), manually type in the coordinates for a known set of coordinates ("Manual") or by locating your spot on a map ("Map"). You can see some examples of using the map view here.

Depending on your location, it can take some time for the iPhone GPS to lock on your location. The iPhone (like most cell phones) uses a system called "assisted GPS" or "AGPS" to quickly get a location lock. This system uses the nearby cell towers to get an initial fix that the iPhone can then use to decide which part of the sky to search for the appropriate GPS satellite to get an accurate fix. If you happen to be out of range of a cell tower, the iPhone needs to progressively search for an appropriate satellite to get a fix from. It starts looking from it's last know location. If you happen to have travelled a great distance since the last time your iPhone used it's location services it may take a long time to search for the right region of the sky (the iPhone only has 1 GPS antenna and it can only listen for one of the 24 satellites at a time and each search can take up to 2 minutes to time out) so the iPhone might time out before it finishes looking for it's location. You can simply restart the GPS and it will keep looking from where it left off and It should find it's location after a couple of tries. Once it knows your general location each subsequent location should lock quite quickly. If you are traveling by air or boat, you can help limit the amount of time it takes to get a lock if you start the location services while you are still in range of a cell tower (either within Dive Log or using the built n Maps application) near to where you will be diving. This will initialize the GPS system in the right part of the world and allow the initial lock to happen quite quickly. The reason that "regular" GPS system can lock more quickly than a phone when they don't have a starting point is that they typically have up to 12 channels and antenna so they can scan the sky much quicker.

I hope that helps. Let me know if you need additional assistance.

Cheers,

Greg

sgatke
Posts: 1
Joined: Tue Aug 09, 2011 12:55 am

Re: GPS co-ordinates

Post by sgatke » Tue Aug 09, 2011 1:10 am

One question about using the inbuilt GPS in the iPhone:
Will I be able to find the coordinates of a dive site if I don't have a cellphone signal?
I thought the iPhone GPS was depending on being able to load the map using some kind of data connection. But perhaps it can find the GPS coordinates without a data connection?

Example, if I am on a liveaboard far from any cell towers but still want to save the coordinates from a dive, will that be possible?

Cheers
Simon

support
Posts: 698
Joined: Wed Mar 31, 2010 4:35 pm

Re: GPS co-ordinates

Post by support » Tue Aug 09, 2011 8:32 am

Hi Simon,

Yes, the iPhone can acquire a GPS location without a data connection. As I mentioned in the previous post, the time to acquire the location the first time can be slow if your iPhone has moved a significant distance since the last time that the GPS function was used (by any application on the iPhone that uses the GPS). For example, if you last used your iPhone in New York before flying to Belize for a live aboard dive vacation out of range of any cell towers or WiFi signals the "cold start" time for the GPS can be quite long. However, if you turn on your iPhone when you land in Belize and have some app (Dive Log, Maps, etc.) determine your location while you are still in range of a cell signal the "cold start" time will be much quicker. Then when you head out on the dive boat the GPS will already have a fix in the "right neighborhood" and will acquire the signal much more quickly. Note that you do not need to establish a data connection for this to work. The iPhone simply needs to see the cell tower signal to get the initial fix that it hands off to the GPS chip (so you can leave data roaming turned off so you don't incur any additional data charges but your iPhone does need to be out of Airplane mode for the GPS to work at all). Even though you can get the GPS location the maps will not be available if you do not have a data connection.

Finally, if you find that the GPS function in Dive Log times out trying to get your first fix, just start it again (you may need to do this more than once) and the iPhone will continue trying to get that initial fix. Once it does, future attempts in the same area will be much quicker as the iPhone will start looking from the last known fix rather than starting with a fix thousands of miles/km away.

Cheers,

Greg

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