Why do I have different bearings?

alric · Occasional Visitor Joined: Jan 10, 2008 Posts: 8

Hello, here's my first post
I'd like to know why I have different bearings for my different systems
I have 5 setups atm,so am deciding which to keep

Here are the readings;
5x.2182/0x.2658 nokia 5500/royaltek gps bluetooth
5x.2182/0x.2658 tomtom on MDA compact2/royaltek
5x.13093/0x.15948 garmin 60csx
5x.13091/0x.15941 memorymap on mda compact2/royaltek
5x.13.6/0x.15561 sony gps-cs1 cybershot

all from the same place,+/-2ft,but basically 2 different readings(how much is the difference?)

mikealder · Posted: Thu Jan 10, 2008 8:05 pm Post subject:

Are you sure all the devices were giving you Decimal.Degree format or were a couple set to Degrees.Minutes format or even Degrees. Minutes.Seconds or possibly something else?
One other thing to check is the datum used, if this differs between platforms you will get different figures - Mike

alric · Occasional Visitor Joined: Jan 10, 2008 Posts: 8

changing the format on the 60csx puts it in line with the others.
It says wgs84 for datum, but there are lots of options
Which should I be using?
I'm in UK

mikealder · Posted: Thu Jan 10, 2008 8:34 pm Post subject:

WGS84 will be fine for your location, I would expect the device to read within 30 feet of each other in use (or less) assuming the coordinate system is consistant under decent weather conditions - Mike

robertn · Frequent Visitor Joined: Feb 06, 2005 Posts: 564

Within reasonable errors of a few feet, These are all varitions of the various coordinte schemes
dd mm ss.s
dd mm.mm
dd.ddddd

Where d = degress, m=minutes and s = seconds.

If you are trying to compare performance, the normal way is time to first fix. What I find even more effective is to get the reciever with a fix, and move it to a location where it is picking up only 3 or 4 satalites. Then you can perform the TTFF test. This way will sort out the boys from the men.

Many devices have diagnostic outputs - if you can be bothered working out what SNR's, DB's, azmiuths and alititudes mean, these are a good guide, but understanding them is lightyears beyond what most people (rightly) care about.

Over all, any modern (last 2 years) chipset is better than you need. The difference between devices is so small you won't notice it in practical use.

Prodnose · Occasional Visitor Joined: Dec 28, 2005 Posts: 42

Hi,
I must confess that I am confused by your presentation of the figures. But I suspect that you have both decimal degrees and degree + minutes. If you take the the decimal part of the first row of figures and convert them to minutes you get
.2658 x 60 = 1.3092
.2182 * 60 = 1.5948
The same digits as in row 3. This can hardly be a coincidence. What I can't explain is what happened to the decimal points in the calculations: they seem to have move up a point in your table. It does add evidence that the other replies are on the right lines.
Harry

Prodnose · Occasional Visitor Joined: Dec 28, 2005 Posts: 42

Hi,
These decimal point are indeed very slippery. My calculations should have read 15.948 and 13.092. Doesn't change the argument.
Harry

philpugh · Posted: Fri Jan 11, 2008 6:15 pm Post subject:

I must say it looks as if you are mixing up the formats .

the first two look klike dd.dddd format whilst the third and fourth look like dd mm.mmm format.

eg
5x.2182 (dd.dddd) = 5x 13.092 (dd mm.mmm)
0x.2658 (dd.ddd) = 0x 15.948 (dd mm.mmm)

Also as lines 1 2 and 4 seem to be measured using the same GPS then it certainly seems to confirm what prodnose and I think.

What the fifth is doing appears to be

5x 13 06 (dd mm ss) and
0x 15 56.1 dd mm ss.

Now I hope these were done outside with a good view of clear sky and left to settle for a few mins. All units should be set to the same datum - I would suggest WGS84 as some devices can't change this.
_________________
Phil

alric · Occasional Visitor Joined: Jan 10, 2008 Posts: 8

actually the last reading should read 5x 13 06.00/0x 15 56.10
the software left a zero out
They were all done indoors with receivers by the window
What is the TIFF test?

and apologies for my ignorance

Prodnose · Occasional Visitor Joined: Dec 28, 2005 Posts: 42

Hi,
Looking again at your original figures it is clear that you have the same position within very close limits but expressed in different formats.
Lines 1 and 2 are degrees in decimal format.
Line 3 and 4 are in degree and minutes decimal
Line 5 is degrees, minutes and seconds decimal
Comparing lines 1 and five we get:
0.2182 (degrees) x 60 = 13.092 minutes
taking the fractional part, 0.092, and converting to seconds
0.092 x 60 = 5.2 seconds
i.e. 5x deg, 13 min, 5.2 secs

With similar treatment, the longitudinal part comes to
x deg, 15 mins, 56.88 secs

Do you follow / agree with the reasoning?

Some instruments allow you to select the format in which the cordinates are presented while other offer "Hobson's Choice".There are co-ordinate calculator available. I can supply one if you are interested.

alric · Occasional Visitor Joined: Jan 10, 2008 Posts: 8

so by your calculations #5 would read 5x.1305/x.1557
Pretty close

I never had to work with different formats before, but I think I understand now
although I'm more interested in parameters such as battery life,easy to read,ruggedness,reliability,and software for remote areas(asia) atm,
I'd be interested to know what robertn is talking about(azimuth,SNRs etc)
as I'm sure the time will come when I have no more than a 40 year old map in some odd format to guide me,and I will need to know how to relate it to what I know already

Prodnose · Occasional Visitor Joined: Dec 28, 2005 Posts: 42

Hi,
Not sure we are quite what you are converting #5 to. So forgive my re-work.
#5 is 5x 13 06.00/0x 15 56.10 which I translate to
Lat 5x deg : 13 min : 6.00 sec
Long x deg : 15 min : 56.10 sec
So, to convert to deg and minute decimal

Lat
5x deg &
13 + (6/60) = 13.1 minutes
Long
x deg &
15 + (56.1/60) = 15.935 minutes

To convert to degrees decimal

Lat
5x deg +
13/60 = 0.21667 deg +
6/3600 = 0.00167 deg
5x.21834 degrees
Long
x deg +
15/60 = 0.25 deg +
56.1/3600 = 0.01558 deg
x.26558 degrees

0.0001 deg lat is approx 36 ft

robertn may prefer to answer himself. In case he dosn't, he is talking about measuring performance. SRN is a measure of signal to noise ratio. A good receiver will have a high SRN. dB is a measure of the satellite signal strength. Azimuth is the horizontal bearing of the satellite and alititude is the angular height of the satellite above the horizon. As robertn observed, your chip set is better than you (we) need.
Finally, I spent as little time trying to work out TTFF and then tried Google. It first lead me to a gay web site in Germany. Finally I got it: Time To First Fix - a measure of GPS perfomance. Looking at Google, "TTFF" seems to seems to be a term used by the gay community. Whether it means the same as in GPS usage I don't know. I shall certainly be careful where I use the acronym in public.

M8TJT · Posted: Sun Jan 13, 2008 8:32 am Post subject:

TTFF stands for Time To First Fix.
This applies when the GPS is first switched on in a different place to where it was switched off. It is also dependent on the time deley between switch on and switch off. (Simplified for the purists) When you firstswitch on the device, it assumes that you are in the same place as when you switched off. It 'looks for the satelites' from that position, if it cant find them because you have moved an appreciable distance, it has to start a new search. The time delay between switch off and switch on has a similar, but not so noticeable effect.

SNR is the signal to noise ratio, and is a measure of how much 'louder' the satelite signal is when compared with the noise (mainly) generated by the GPS device itself (and other sources) .

mikealder · Posted: Sun Jan 13, 2008 11:26 am Post subject:

You might want to have a read through This Page for an explaination of terms associated with GPS and how it all works - Mike

robertn · Frequent Visitor Joined: Feb 06, 2005 Posts: 564

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