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I checked this out and it came out correct because my ISP server is in the same city as me. Be aware that magnetic declination changes over time as the magnetic poles wander. About 24 years ago my magnetic declination was at 0.0 degrees, but it is currently at 3 degrees, 31 minutes West.

SatPhreak said:

I checked this out and it came out correct because my ISP server is in the same city as me. Be aware that magnetic declination changes over time as the magnetic poles wander. About 24 years ago my magnetic declination was at 0.0 degrees, but it is currently at 3 degrees, 31 minutes West.

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SatPhreak,

Good point!

Here is another tip... If you have a good GPS unit, you don't need to worry about this angle at all. Use true positioning (satellite triangulation) and you can forget about any magnetic deviations to correct your compass reading. Any anomalies with metal buildings, high tension power lines or localized mineral (ground) deposits will have no affect.

Radar

AcWxRadar said:

SatPhreak,

Good point!

Here is another tip... If you have a good GPS unit, you don't need to worry about this angle at all. Use true positioning (satellite triangulation) and you can forget about any magnetic deviations to correct your compass reading. Any anomalies with metal buildings, high tension power lines or localized mineral (ground) deposits will have no affect.

Radar

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I have a newer GPS unit, and a Garmin GPS III which I used before I got the new one. I wired it into my laptop with the data cable and ran Streets & Trips when I had a job that required me to drive to and fro around customer locations.

Both of these only work while you are MOVING! So, I cannot use them to point my dish.

glen4cindy said:

I have a newer GPS unit, and a Garmin GPS III which I used before I got the new one. I wired it into my laptop with the data cable and ran Streets & Trips when I had a job that required me to drive to and fro around customer locations.

Both of these only work while you are MOVING! So, I cannot use them to point my dish.

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I agree. I think part of the confusion with respect to bearings and GPS is that some GPS units must have built in electronic compasses, which have nothing to do with GPS, and thus you'd still have the same issues with local magnetic anomalies, metal pipes, etc. If it's just a GPS, then absolutely, you cannot get a bearing just standing there.

I've read posts from people who agree with this, but claim that what they do is to walk rapidly away from the dish while watching the GPS, and read the heading indicated. Putting aside the fact that you'd have to perform several of these walks to figure out the direction you're looking for, and would have to walk a very long distance to get a reasonable estimate of heading, I really don't think that this can be very accurate, however it's *possible* that this could get you close.

The reason I was doubtful about the accuracy is that with most consumer GPS units, you're lucky to be able to get accuracy to +/- 50', even though when you read the estimate of accuracy that the GPS gives, sometimes you'll see things like 15' or some units suggest even better accuracy, in the 3-5' range.

I have done MANY experiments with the 6 or 7 GPS units I've owned over the years, and while I know that the newer GPS units might be capable of more accuracy than the older units, these things just aren't as accurate as people think.
Most of my experiments have involved connecting the GPS to a computer and recording the NMEA messages sent out by the GPS, and plotting the indicated position over time. One experiment involved using a fixed external GPS antenna, collecting data for over day or more. I've done this on older GPS units with/without SA , and have also tried it on WAAS units and I also have a DGPS adapter. Every time I've tried this, I was really dissappointed at the lack of accuracy. The older units would wander HUNDREDS of feet, even though they are rated to 50' without SA. Both the WAAS and LF DGPS units are rated down in the range of less than 10', however even with those, I see wandering of 50-100' at times. I've also tried to do simulated DGPS with two GPS units, using one as a reference, but that was even worse, because with consumer GPS units, you can't insure that the 2 units are using the same sats for the calculation.
I have also mapped some trails around my house DOZENS of times, with the WAAS and DGPS units, and the narrow trails appear to be 100' wide in the combined plots. So the accuracy of GPS cannot be relied on.

HOWEVER, actually, accuracy isn't necessary. For getting a heading while moving, accuracy isn't important, but precision is. Ie you get the heading by comparing two positions, and seeing which direction the second position is relative to the first. Both positions can be off by a mile, as long as the relative orientation of the two is correct. Ie slow wandering wouldn't affect the accuracy of GPS headings, so it's possible that headings obtained by GPS MIGHT be quite accurate. I've never seen any specs relative to the short term precision of GPS measurements related to the accuracy of headings, but I have done some crude experiments walking down my driveway at a fast pace multiple times, comparing the headings, and I wasn't very impressed by the precision of the headings indicated. I was getting bearings that varied from 295 to 320, even on the same trial. Clearly not good enough to be useful, unless you collected the data and averaged the results.

Anyway, while I LOVE GPS, and use them for all sorts of things, unless you have a very expensive professional DGPS unit, I don't think it can be relied upon to give bearings that are any better than a cheap compass, in particular a compass where the needle moves, not the whole scale. However my opinion is based on the GPS units I've owned, so there may be newer units out there that perform better.

BTW, my experiments with my DGPS unit were somewhat amusing. The DGPS option is a box, about 3x5x8" or so, and has an antenna similar to a GPS antenna, and it fed into my regular GPS, with which I was using an external antenna that looked like a 5" tall egg. Mounted both antennas on a fork shaped PVC tubing holder, and put everything in a back-pack, with the antennas sticking out the top over the back of my head. I looked like a Ghost-Busters character walking around the neighborhood. Luckily we don't get very much traffic on our road, so not too many people saw me.

But after seeing the poor results, I decided that the contraption wasn't worth the effort.

I have used a GPS many times to set a very close direction.
First, I go to my dish location and set a waypoint. Next, I go out about 100 feet and get a "Bearing" reading for the waypoint on the GPS. This is 180 degrees out from the heading, but is easy to calculate. I then put up a marker at the bearing I want, go back to the dish and sight to my marker.
I was off only one time, and I may not have done my math right. Otherwise I have been within "tug" range. BTW magnetic readings here are worthless. The sun rises in the north east on the center of our property in the east at the north side and in the south at the real south end of the property. A lot of minerals in the ground and a major power line From The Dalles Dam running across it! Also very windy, but would that affect a compass?

Wind won't affect a compass.