znod

Hi Friends:

As I recall, my manual indicates that the accuracy of the 5-Series altimeters is limited--as one would expect. Naturally, given the usefulness of the index in my manual, I can't locate the information again--with reasonable ease.

Does anyone have info, or an educated guess about, the relative accuracy of our altimeters. I emphasize that I am not overly interested in their absolute accuracy. In this regard, close is good enough for me. What I want to know is how likely it is that the altitudes of two places shown to be the same by my altimeter really are the same.

DD694

I was once told to look at GPS positions as being somewhere inside a sphere of a diameter equal to the location accuracy of the system. Hence, if the positional accuracy is within 30 feet, imagine a sphere 30 feet in diameter. You are somewhere inside that sphere.

znod

I was once told to look at GPS positions as being somewhere inside a sphere of a diameter equal to the location accuracy of the system. Hence, if the positional accuracy is within 30 feet, imagine a sphere 30 feet in diameter. You are somewhere inside that sphere.
[/quote]

Thanks much. So, if I understand correctly, then you are saying that the altitudes of two places, having produced the same altitude reading, would be within 30 feet of each other's altitude if the positional accuracy of the altimeter used is 30 feet. And, I wonder how good the positional accuracy of our cars is. Back to trying to find something in the manual. Thanks again.

stream

The GPS system cars use is the civilian GPS system, which is intentionally limited in its accuracy, for defense reasons. The secure GPS system is very accurate.

tachyon

Znod -

I only have some anecdotal evidence about the accuracy of the altimeter in the E60 GPS system, relative and absolute.

I sometimes drive through a ravine on Sheridan Road north of Chicago that dips down low as it hugs Lake Michigan. The elevation shown on my iDrive is 675 feet at the top of the ravine, and it reads between 575 and 590 feet at the bottom.

I would guess that the actual drop-off really is between 75 and 100 feet. So the relative reading seems accurate enough.

And the Encyclopedia Britannica online states that the elevation of Lake Michigan is at 575 feet. So the absolute reading also seems about right.

znod

Hi stream:

Thanks. Do you have any insights concering the relative accuracy of a civilian GPS?

DD694

Thanks for the help tachyon. I could not find the accuracy info I mentioned by looking briefly at each page of my manual. I am beginning to think the info is one of the booklets that came with my wife's X5--which has a separate nav booklet. I'll look there, but, as I recall, the statement is something pretty much like "the altitude measures are not necessarily very accurate."

Hi stream:

Thanks. Do you have any insights concering the relative accuracy of a civilian GPS?
[/quote]
In my previous example the maximum error would be 60 feet - 30 feet plus 30 feet. Top of one sphere to bottom of other sphere.

The government eliminated dithering(Selective Availability) in May 2000 of the GPS positional information for civilian use. Prior to that date accuracy was from 15 meters 95% of the time to 100 meters 95% of the time. Now with SA disabled the accuracy is around 8 meters. They can remove the GPS signal completely at any time they deem it necessary for national security. This is one reason that the EU is working to develop their own independent GPS system. Today using Differential GPS provides accuracy to 6 meters. WAAS GPS will provide accuracy to 3 meters. Either of these are avalaible in marine fixed mount GPS. Differential GPS (DGPS) is available in handheld GPS devices.

znod

In my previous example the maximum error would be 60 feet - 30 feet plus 30 feet. Top of one sphere to bottom of other sphere.

The government eliminated dithering(Selective Availability) in May 2000 of the GPS positional information for civilian use. Prior to that date accuracy was from 15 meters 95% of the time to 100 meters 95% of the time. Now with SA disabled the accuracy is around 8 meters. They can remove the GPS signal completely at any time they deem it necessary for national security. This is one reason that the EU is working to develop their own independent GPS system. Today using Differential GPS provides accuracy to 6 meters. WAAS GPS will provide accuracy to 3 meters. Either of these are avalaible in marine fixed mount GPS. Differential GPS (DGPS) is available in handheld GPS devices.
[/quote]Thanks. I thought your example might imply 60 feet.' Do you think the difference would be completely independent of the distance between the two points being measured--using the same altimeter? For example, what if I took an altitude reading on one side of a hill, drove to the other side of the hill, and obtained the same reading--having actually moved only a small distance from the original point--say 1/4 or 1/2 mile. In contrast, what if I obtained the same readings having driven from one town to another--say 10 miles apart?

DD694

In my previous example the maximum error would be 60 feet - 30 feet plus 30 feet. Top of one sphere to bottom of other sphere.

The government eliminated dithering(Selective Availability) in May 2000 of the GPS positional information for civilian use. Prior to that date accuracy was from 15 meters 95% of the time to 100 meters 95% of the time. Now with SA disabled the accuracy is around 8 meters. They can remove the GPS signal completely at any time they deem it necessary for national security. This is one reason that the EU is working to develop their own independent GPS system. Today using Differential GPS provides accuracy to 6 meters. WAAS GPS will provide accuracy to 3 meters. Either of these are avalaible in marine fixed mount GPS. Differential GPS (DGPS) is available in handheld GPS devices.
[/quote]Thanks. I thought your example might imply 60 feet.' Do you think the difference would be completely independent of the distance between the two points being measured--using the same altimeter? For example, what if I took an altitude reading on one side of a hill, drove to the other side of the hill, and obtained the same reading--having actually moved only a small distance from the original point--say 1/4 or 1/2 mile. In contrast, what if I obtained the same readings having driven from one town to another--say 10 miles apart?
[/quote]
Yes, I think the measurements would be completely independent of each other. The sphere is determined at a particular point in time that I assume depends upon the position of the satellites relative to you and which satellites are being used at a given moment. The newer GPS systems utilize up to 12 satellites.

znod

In my previous example the maximum error would be 60 feet - 30 feet plus 30 feet. Top of one sphere to bottom of other sphere.

The government eliminated dithering(Selective Availability) in May 2000 of the GPS positional information for civilian use. Prior to that date accuracy was from 15 meters 95% of the time to 100 meters 95% of the time. Now with SA disabled the accuracy is around 8 meters. They can remove the GPS signal completely at any time they deem it necessary for national security. This is one reason that the EU is working to develop their own independent GPS system. Today using Differential GPS provides accuracy to 6 meters. WAAS GPS will provide accuracy to 3 meters. Either of these are avalaible in marine fixed mount GPS. Differential GPS (DGPS) is available in handheld GPS devices.
[/quote]Thanks. I thought your example might imply 60 feet.' Do you think the difference would be completely independent of the distance between the two points being measured--using the same altimeter? For example, what if I took an altitude reading on one side of a hill, drove to the other side of the hill, and obtained the same reading--having actually moved only a small distance from the original point--say 1/4 or 1/2 mile. In contrast, what if I obtained the same readings having driven from one town to another--say 10 miles apart?
[/quote]
Yes, I think the measurements would be completely independent of each other. The sphere is determined at a particular point in time that I assume depends upon the position of the satellites relative to you and which satellites are being used at a given moment. The newer GPS systems utilize up to 12 satellites.
[/quote]Great info. I really appreciate your help. I have been "observing" some of what you say looking at meters and GPS devices on the REI site. Off hand, do you know how accurate watch-type altimeters would be? If you want to think about it, here is a link to one I might be interested in. It also has functions for other things I am interested in. Thanks much whether or not you feel like continuing.