One of my GPS receivers writes my longitude as -284 degrees. I live between New York and Washington and I think the receiver should say either -76 or 284, not -284. It’s as if it is expressing westness twice, which makes it eastness. Some software interprets these points as in China, some puts them where I am, and some flags it as an error.
What are the rules for expressing longitude? Is there a precedent for making the angle bigger than 180 AND changing the sign? Is there some customary weirdness peculiar to longitude, some artifact of antiquity? Or is this receiver junk?
Longitude is properly expressed as a number between -180 and 180.
Adding or subtracting multiples of 360 yields the same mathematical result, so -76 and 284 can be considered equivalent - but the latter is certainly not a normal way of expressing longitude.
-284 is both non-standard and not equivalent to -76, so is clearly wrong here. I’d say your receiver is highly suspect.
I have used coordinate systems that use negative longitudes > than 180. Not often, but sometimes. They are usually used to support computer programs that find such negative numbers easier to handle (no checking <> 180 required), not so much for mere mortals.
So, it is an accepted technique and it is clear what is going on, but it isn’t usual.
Are you sure you’re using NAD83 coordinates and not some sort of state plane coordinate system? Some of the older GPS units you could change what datum you wanted. If that’s the case you could be set up for a state further east then you are which could give a negative number.
I don’t remember all about the state plane coordinates, but usually they started in the lower east corner so all numbers would be positive.
It’s also quite normal - and indeed agreeably simple - to express it as a single number (which includes a sign). In the past it was common to express it as four quantities: hemisphere, degrees, minutes and seconds. This is clear enough - but it’s cumbersome, and is now falling out of favor.
Thank you for the correction. My question to you as a professional is, is your sign convention (+ = East, - = West) universal enough so there is no chance someone would get it backwards? What about a someone in a slightly different profession, like an astronomer or a sailor?
Let’s observe that the OP’s GPS seems to consider west longitude as positive, for whatever reason. I know that GPS does, or used to, use WGS 84 coordinates, but does that or related standards explicitly mention the sign convention? I also would have guessed longitude to increase towards the east, since the planet rotates that way.
I don’t think the Earth’s rotation has a lot to do with it.
Starting at (0, 0), Cartesian coordinates increase as you move right and up, and go negative as you move left and down. It seems sensible that lat/lon coordinates should do much the same thing.
Cartesian coordinates describe points in a plane. The surface of the Earth is, to a reasonable approximation, an ellipsoid. As for orientation, note that for a rotating planet you have to decide which is the north pole and which is the south pole.
The convention is that anything east of the Prime Meridian is positive, West is negative, hitting +/-180 at the date line. To my knowledge, anybody that works with lat/long regularly will be aware of that, as the advantage to using signs is for doing geodetic calculation work (which many sailors and astronomers may be doing).
Positive east/north is definitely the usual notation for mapping out in Cartesian coordinates, but it is definitely not universal. Some regions of the world use other systems, and occasionally some projects might be set up reversed even in the United States.
Surveyors also often set up Cartesian systems that begin at a greater-than-zero origin, to eliminate the possibility of negative numbers. My company standard is to set our origin point at N:50,000/E:50,000. We do large-scale work, with survey areas that can be 60 square miles (or larger).
I study ocean marine biology in the Pacific where we do transects across the dateline, and I spent lots of time converting +/-180 in my data to 0-360, so all my graphs aren’t discontinuous. (well, by “lots of time” I mean getting a messed-up plot the first time I plot any new data, sighing and putting in the function).
This turned out to be the whole problem. When I set it right side up, everything looked just fine.
Ahhh, if it were only that simple.
Nothing works right – including multi quote for some reason today. But to address a few points:
My receiver doesn’t consider west longitudes to be positive. It considers them to be negative, but much more negative than -180.
The values I am getting do not work correctly for the idea that I can add any integer multiple of 360 and get the same result. I think some software treated the numbers this way, which is how I wound up hiking in China.
It’s interesting, one software package treats the numbers as valid (but in China) when using one feature of the software, but invalid when using another feature of the software.
I had about 20 track files (GPX) with this problem. These are simple text files, though they make monotonous reading. So, using a text editor, I did a search-and-replace on all of them, replacing the string lon="-28 with the string lon="28, and now, happily, all the software I’ve tried treats them the way I want, bringing my hike back to North America.
