I went for a walk (partly along a beach) yesterday. It was tracked with 2 different GPS receivers.
Checking today I noticed that at one point on the beach, one track said we were 20 feet below sea level, and the other 20 feet above.
Altitude is not accurate on consumer grade GPS.
Good story, but a fake alas:
tie some string to the barometer and throw it over the building. pull the string taunt at equal distances either side so it makes an equal sided triangle and do some maths.
The message from Bruce contains these words:
No, the vertical accuracy is approximately half the horizontal accuracy. This is due to the angles used in the calculations rather than any intrinsic difference between the two dimensions.
FWIW the quoted positional accuracy for my current handheld GPS is 15 metres 95%. ISTR that an earlier example quoted the accuracy as 95% RMS which seems to make a bit more sense than just sticking a percentage after the quoted figure.
Get a long length of timber and stand with it one end on the floor behind you and with it resting on your shoulder with a length sticking out in front that you can sight along. Move yourself and the timber until you can sight along its edge to the top of the building.
Measure the distance between the end of the timber behind you and the centre of your feet. T Measure the height of your eyes. E Measure the distance to the Building B
Height of building = E * B/T
I've never tried this, but in principle it should work, Also, so long as the ground is sloping evenly, you don't even need to do this on level ground.
Are you sure it's not a feature-poor version of a theodo?
No, it is nowhere near that accurate in the vertical dimension.
I repeat, the resolution of height using a GPS receiver is an order of magnitude less accurate than its resolution of plan position.
No - I'm not. :-)
Couldn't find it in several searches - so only exists in my very imperfect memory.
The message from Bruce contains these words:
Now that is a very bold statement, particularly when it is not backed up by any detailed reasoning.
You can repeat your statement as often as you like but it won't make any difference. If you are wrong in the first place you will remain wrong and the fact that you will not even countenance the possibility that you are wrong casts doubt on all the many confident assertions you make on this ng.
Anyway I look forward to your explanation of how a machine that determines a position in space can expect an error on the z axis 10 times as large as the combined errors of the x and y axes.
And don't try and build a case on the difference between the Geoid and the Ellipsoid, it just won't wash.
Not only bold, but correct. If you wish to disprove it with detailed reasoning and references to authoritative articles in peer-reviewed scientific journals, don't let me stop you.
The message from Bruce contains these words:
You wish. It is open to you to explain but you chose not to.
There is plenty of information on the Internet that backs up what I said and nothing at all as far as I can see that backs you up. Seems to me that you don't really have a clue and are hoping I can't be bothered to dig out something from science journals I have to pay to view. You are quite right, I can't be bothered but you still lose. I shall take everything you say in the future with a large shovel full of salt.
At this point, Harry Hill would say "Fight!"
MBQ
The manual for my old Garmin stated that the vertical position error was vast compared to the horizontal position accuracy. I can't see them lying about it.
In message , Roger writes
Have to say that the claim of an order of magnitude different doesn't feel right, nor was it something I came across when researching the purchase of a new GPS unit recently.
Thought he level of inaccuracy is such they would indeed not be much use for anything where any sort of accuracy is required.
Though mine has a barometer to use for altitude measurements.
Did it really take two replies for you to realise that?
So you expected to see a statement that "This GPS unit is rubbish at determining altitude"? Obviously you know very little about marketing. If everything we bought came with truly honest information about its limitations, we would buy much less of it.
My point, exactly. A protractor, spirit level and tape measure would be far more reliable and accurate.
Didn't it occur to you that it has a barometer because GPS is rubbish at determining altitude?
Bruce is quite right that the vertical accuracy is somewhat less (although I wouldn't say an order of magnitude) than the horizontal accuracy. This is just a matter of geometry.
It is much the same issue as trying to fix your position at sea by taking bearings on landmarks: if you select two which are close together or nearly 180 degrees apart then the accuracy of your fix suffers. The GPS satellites are rarely directly overhead: so in the vertical you suffer from dilution of precision.
There are some calculations that can derive this and generate a plot against time for whatever location on the globe you are. It's all fairly well known, google for VDOP or HDOP.
The message from Jim contains these words:
Dennis lives in my kill file so when I am home I only see what he says when someone else reposts it. Like much of his output there is a element of truth in what he says but misintepreted to the nth degree. I never bothered to get a GPS till SA was turned off. With SA the positional accuracy was degraded to some 100 metres so with the vertical accuracy being a factor of 2 worse in the UK the height output was useless for all practical purposes.
No Bruce is wrong.
The message from Bruce contains these words:
No, I was just trying to be polite. The truth is you haven't the evidence to support your claim nor the guts to admit you were wrong.
How can it be misinterpreted, it is a statement of what it said with no interpretation at all! Some people will argue about anything!
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