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Boat Forum / Cruising / April 2005Islamic GPS and direction finding
I wuz playin with my GPS when I glanced over at the globe and starting thinking (always a dangerous thing). Wouldnt it be interesting to set up a GPS to always point out the direction of the shortest distance to a given point on earth from your location. Note that on a sphere, this is always a great circle so isnt always obvious when looking at a flat map or sometimes even when looking at the globe. At first, this was just an abstract thought with no application but then I remembered that there are people who say they need to face toward a special place each day, muslims face toward Mecca to pray. Not being too familiar with Islam I dont know if they really take the direction they pray seriously but perhaps its just a matter of proper marketing. I cant really think of other applications where a person needs to know the right direction (and distance) to a known point(s) on earth but there must be others. However, on further thought I realized that GPS cannot do this. A single GPS receiver cannot determine the direction you are facing although it can determine the direction you are moving. You would need two GPS receivers set a sufficient distnce apart to get an accurate position of each, probably several meters apart. So, how else could you do this? A compass will not work because the direction it points varies wildly with positiona lthough this could be corrected with an internal database of corrections. Facing Mecca typically means facing East. If somebody needs a GPS to figure out which way East is they probably need a drool cup as well > Facing Mecca typically means facing East. Does that mean that Pakistanis use long path propagation? ;-) Sorry, I couldn't resist.  SignatureGlenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com Not true. I have worked with a number of Muslims. They have a guide that tells them which way to face Mecca wherever they happen to be in the world. Their biggest problem is determining when sunset is while doing world travel. Doug > Facing Mecca typically means facing East. > > If somebody needs a GPS to figure out which way East is they probably > need a drool cup as well > Not true. I have worked with a number of Muslims. They have a guide that > tells them which way to face Mecca wherever they happen to be in the world. > Their biggest problem is determining when sunset is while doing world > travel. A consumer-level GPS could help with that as well. All the ones I've seen include sun/moon rise/set times although some of the algorithms run into problems when in the arctic or antarctic regions. >>Facing Mecca typically means facing East. >> >>If somebody needs a GPS to figure out which way East is they probably >>need a drool cup as well >> Not true. I have worked with a number of Muslims. They have a guide that >> tells them which way to face Mecca wherever they happen to be in the [quoted text clipped - 5 lines] > include sun/moon rise/set times although some of the algorithms run into > problems when in the arctic or antarctic regions. Their delemma was that they were airborn. The sun didn't set for the entire trip. >>>Facing Mecca typically means facing East. >>> >>>If somebody needs a GPS to figure out which way East is they probably >>>need a drool cup as well >Facing Mecca typically means facing East. And in Indonesia, home of the world's largest population of Muslims? >If somebody needs a GPS to figure out which way East is they probably >need a drool cup as well Something similar occurred to me during the recent three-week Popeathon, when I wondered why so many guys down on homosexuality dressed like drag queens...but I digress. I believe that Muslims, whose culture did a great deal to develop cartography and navigation ("azimuth" anyone?) are quite aware that "East" doesn't cut it and that it is part of the Muslim clergy's job to provide a reasonable bearing to Mecca from, say, South London, which would be off the top of my head 115 degrees T or so. Another use for the GPS would be the almanac function, as many Muslim religious days begin with the sighting of the very new moon *from Mecca*, and fairly simple calculations from the old celestial nav book and the numbers from even basic GPSes can give you that. R. Perhaps I'm missing your point, but any GPS can know the direction and distance from where it's being used, as long as the coordinates for the other place is entered. The other place, in essence, is the second point, and can be considerably more than yards away. Facing mecca does NOT mean facing east if you are due north of it etc. Yes, with GPS, you can find the direction from one point to another. BUT, without a means to determine that direction (a compass, etc), that is useless. We all know of compass variation etc so I would like something better, maybe a built in gyroscope. A GPS can tell you if you are moving toward a specified point (say a waypoint) but if you are stationary, it cannot determine what direction you are facing. > Facing mecca does NOT mean facing east if you are due north of it etc. > [quoted text clipped - 5 lines] > waypoint) but if you are stationary, it cannot determine what direction > you are facing. A few years ago I actually did enter Mecca as a waypoint into my eMap GPS after seeing a few people conducting an Islamic prayer and being curious as to whether they were using great circle, rhumb line, or some other choice of direction. There's no need for a second GPS or gyroscope since a single GPS unit can give you the great circle bearing and distance using your choice of 'true north' 'grid north' or 'magnetic north'. So if you set it to 'magnetic north' you can then use a regular compass to immediately see the correct great circle direction. Many consumer GPS units even include a magnetic compass so they can point in the correct direction while stationary (e.g. the Garmin Summit, Vista, 60/76cs, 76s, Geko301 and the Magellan Platinum, 300, and 600). And for other models just taking a few steps in a straight line will give them the necessary orientation to point in the right direction. BTW, my observation was that although Mecca is at a bearing of 19° from my location on a great circle route, the people praying were generally facing much more nearly east (bearing 90°). You can do it with one GPS. Just walk across the Mosque with the GPS on and the highway display will tell you which way to lay out your rug. :-) SignatureGlenn Ashmore I'm building a 45' cutter in strip/composite. Watch my progress (or lack there of) at: http://www.rutuonline.com Shameless Commercial Division: http://www.spade-anchor-us.com > I wuz playin with my GPS when I glanced over at the globe and > starting thinking (always a dangerous thing). Wouldnt it be [quoted text clipped - 18 lines] > because the direction it points varies wildly with positiona lthough > this could be corrected with an internal database of corrections. > I wuz playin with my GPS when I glanced over at the globe and > starting thinking (always a dangerous thing). Wouldnt it be [quoted text clipped - 18 lines] > because the direction it points varies wildly with positiona lthough > this could be corrected with an internal database of corrections. At satellite freqs in the gigs, an inch between 3 antennae would allow direction and attitude finding of the satellites' signals, and with positions and azimuths known, the almanac of orbits would tell a processor enough to determine which way the antenna base on the rx was pointing, and then it could generate a compass needle image, or indicate the direction pointed by the instrument, in magnetic or celestial. Gps today can indicate altitude, and some, I gather, indicate barometric pressure adjusted for altitude, and trends. Some phones include gps and internet, and may become implantable, free, even mandatory in dogs and children. The required great cirle course heading would not be a constant, and could vary considerably all along the way, unless it happened to be a north or south course, not passing over the pole. I believe different Islamic schools face either east or to Mecca, using the direction of sunrise if no other direction is indicated. Terry K > > I cant really think of other applications where a person needs to > know the right direction (and distance) to a known point(s) on earth [quoted text clipped - 6 lines] > because the direction it points varies wildly with positiona lthough > this could be corrected with an internal database of corrections. Silva has produced this for years, don't ask me how it works: http://www.silva.se/pg/products/com_mecca9.htm |
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