Understanding Alerts vis-à-vis POI Coordinates


Understanding Alerts vis-à-vis POI Coordinates

"Alerts" are one of the most misunderstood aspects of a GPS. Many new user download some POI file and load it onto their GPS using POI Loader expecting that they will get an "alert" (either a "bong" or a sound file) whenever their GPS ventures close to one of the locations in the POI file. Most of the time, they are disappointed and confused when "alerts" do not occur.

What most users do not realize that so-called "proximity" POIs will not alert unless they are navigating "along a route" - that is, they have placed a "Where To?" into their device and are proceeding to it. And, even if the user is navigating "along a route", they may not get an "audible alert". Indeed, they may be able to visually see a POI and still not get an "alert".

Perhaps the best place to start when trying to understand "Alerts" is with POI files themselves. Why? Because the placement of the coordinates for a specific POI location has an impact on whether or not one will receive alerts.

Let's think for a minute about how a POI file gets created in the first place. On this site, at least, a member decides that he or she will spend the time to research some business and create a POI file for it.. In the best case, the creator will be able to contact the business and get a complete listing of all of their locations with information like store number, complete address, phone, etc. And, in the really best possible case, the member will be given coordinates, too. However, most times the creator will just have to use the internet to find store locations - laboriously cutting and pasting information from the web site to, say, Notepad or Excel. Each case is different.

If the information the creator gets has coordinates, the hardest part of producing a POI file is done. Otherwise, the creator must "geocode" addresses to obtain coordinates for that address. While there are various ways this can be accomplished, I prefer to use Extra POI Editor (EPE) which was written by a poi-factory member named TurboCC.

But, just because one has an address, does not mean that it can be turned into a set of coordinates - regardless of what tool one uses to geocode. It takes time even for superb tools like Google to access data from jurisdictions on addresses in new developments or new sub-divisions.

Further, having an mailing address does not mean one has a exact "store location" address - think malls.

Suffice it to say that the process of turning an address into a set of coordinates is fraught with problems. I have found that the only way to be sure that you have proper coordinates for some POI is to check them with GoogleMaps. Even then, one is dependent on when the Google images were last updated.

Once a POI creator has turned addresses into coordinates, a determination must be made as to where these coordinates are relative to, say, the "front door" of the POI. My feeling is that most GPS users just assume that the "front door" is where the coordinates will be set.

From the perspective of the creator of a POI file, just having a reasonably close set of coordinates for the location may be all he or she is after. Because, if the purpose of the POI file is to allow a user to navigate to a specific POI - think "find me the nearest Walmart" - then it does not matter exactly where the actual coordinates are so long as one can see signs to the location or the location itself. In the ballpark is close enough for most people.

Herein is the rub. The ability to get an "Alert" for a POI while not actually navigating "to" that specific POI is dependent on whether you want the "Alert" to occur when you are "along the route" or whether you want to be notified whenever you get somewhere near a POI - whether you are navigating a route or not.

The best example of an "along the route" alert may well be the "Rest Areas Combined" file (now being maintained by Vito and previously by MrKenFL). Take a look at http://www.poi-factory.com/node/6643. In the explanation is this statement:


The coordinates shown, in the great majority, are the coordinates for the entrance to the "location" off ramp.

The reason this is important is that "along the route" alerts will occur ONLY when the coordinates of a POI are within 30 meters of the centerline of the road on which you are traveling. Here is a Garmin FAQ I saved some time ago:


How close do I have to be to a proximity point before my nuvi will alert me?
While navigating, the proximity point will need to be within 30 meters of the route, regardless of the distance set in the POI Loader, to trigger an alert. This helps to reduce false alerts that can occur if you are traveling on a road close to a proximity point even if it is not affected by the alert.
Last modified on: 03/10/2011

Note that the distance is specified in meters because the metric system underlies the Garmin software. And, 30 meters is about 98 feet.

So this is why Vito is placing Rest Area coordinates for the entrance to the "location" off ramp (as opposed to the door to the restrooms) and also suggesting that you set a proximity in manual mode of POI Loader to 1 to 2 miles so you will have time to make a decision as to whether or not to stop at the next Rest Area.

Now, just to muddy the water a bit, let's think about those Rest Areas that on either side of a divided highway - directly across from one another. There are several of these close to me such that I get an alert for both of them, even though I can only turn into one of them because of the divided highway. Likewise, there are some Rest Areas where you will be alerted only to find that this particular Rest Area is on the opposite side of the divided highway on which you are traveling.

How would this occur? Well, the coordinates for such a Rest Area will turn out to be within 98 feet (30 meters) of both sides of the divided highway. The only way Vito would know of any of these instances would be to be notified by one of us - which I try to do when I find them. My point in mentioning this is to emphasize the fact that "Alerts" can produce quite unintended results.

Before one uses any POI file for "Alert" purposes, one must determine whether or not the creator/maintainer of the file had any specific purpose in mind when determining which coordinates to use.

For example, I was asked to take over maintenance of Jo-Ann Fabric and Craft stores. I got a complete list of stores from the Jo-Ann corporate offices and proceeded to "geocode" them. I then looked at each location using Google so that I could be able to say the following in http://www.poi-factory.com/node/19280.


The coordinates have been placed at an intersection into the (usually) strip malls in which these stores are located. Whenever possible, I have tried to pick entrances that also contain signage indicating that there is a Jo-Ann store in the mall (whenever I was able to find such using Google street view)

Now, I do not expect that many people would want an "Alert" to occur for a Jo-Ann. But maybe this would be a scenario. Say, I am out-of-town at a conference that will last several day and I have my wife with me. There will be enough free time for us to go places during the day. I might have said to myself - if ever I am on my way somewhere - say to eat - and there is a Jo-Ann along the way on the "route" to the restaurant (or wherever I am routing to), I want to be notified so we might be able to stop there on the way back to the hotel after we finish eating.

A discussion of "Alerts" would not be complete without a discussion of "TourGuide". One POI that many of us use as an example of why we like "TourGuide" is Cracker Barrel. When traveling, this is always a good place to eat. You know just what to expect when you eat at any of them. So, many of us use "TourGuide" in the name of our Cracker Barrel POI file - see http://www.poi-factory.com/node/17225 - and set a proximity of several miles. Then, whenever the GPS device comes within an "as the crow flies" distance of 2 miles of any Cracker Barrel (whether we are on a "route" or not), an "Alert" will occur. Now bear in mind that this particular Cracker Barrel that just "alerted" you might be on the other side of a, say, river, and be extremely difficult to get to from where the GPS device is currently. Such a situation is just another of those unintended consequences of "Alerts".