Lon/Lat Locations

Discussion in 'General Fitness' started by Geoff Lane, Jan 10, 2005.

  1. Geoff Lane

    Geoff Lane Guest

    Are there any other sites other than www.streetmap.com that will give
    a location if you enter a map co-ordinate?

    Reason I ask is that I have a Garmin Etrex and I have often used it to
    get to a location by entering the postcode on Streemap.com which then
    gives me numerous map references - last week I used it to go to a
    location near Croydon and when I neared my destination it was advising
    a left turn where in fact I needed to turn right.

    The Garmin's data showed it to be about a mile wrong so I'm thinking,
    either;

    1. The Garmin is wrong (Doubtful)
    2. Streetmap.com's data may be incorrect (possible but not sure).

    On reaching the location I took an actual reading from the Garmin and
    they are as follows;

    Incorrect N51:20:10, E0:0:16
    Actual N21:21:00, E0:0:33

    The incoorrect reference shows on streetmap.com as in Layhams Road but
    my Garmin was suggesting West of Henrys Drive into Fairchilds Avenue.

    I appreciate there are inaccuracies in GPS but this was telling me a
    deffinite left was needed whereas it was actually a right turn.

    Any comments or suggestions welcomed.

    Geoff Lane
     
    Tags:


  2. Rooney

    Rooney Guest

    On Mon, 10 Jan 2005 18:14:25 +0000, Geoff Lane
    <[email protected]> wrote:

    >Are there any other sites other than www.streetmap.com that will give
    >a location if you enter a map co-ordinate?



    Microsoft's Autoroute will do this, if that's any help.

    --

    R
    o
    o
    n
    e
    y
     
  3. Geoff Lane

    Geoff Lane Guest

    On Mon, 10 Jan 2005 18:17:01 +0000, Rooney <[email protected]> wrote:

    >On Mon, 10 Jan 2005 18:14:25 +0000, Geoff Lane
    ><[email protected]> wrote:
    >
    >>Are there any other sites other than www.streetmap.com that will give
    >>a location if you enter a map co-ordinate?

    >
    >
    >Microsoft's Autoroute will do this, if that's any help.


    Forgot about that one - thanks.

    Geoff Lane
     
  4. In article <79g[email protected]>, Geoff Lane
    <[email protected]> writes
    >Are there any other sites other than www.streetmap.com that will give
    >a location if you enter a map co-ordinate?


    Multimap allows you to search for postcodes and display them on a map.
    You can also click on the map to recentre it and then read off the
    co-ordinates below the map.

    http://www.multimap.com/map/browse.cgi?lat=51.3355&lon=0.0012&scale=10000
    &icon=x
    >
    >Reason I ask is that I have a Garmin Etrex and I have often used it to
    >get to a location by entering the postcode on Streemap.com which then
    >gives me numerous map references - last week I used it to go to a
    >location near Croydon and when I neared my destination it was advising
    >a left turn where in fact I needed to turn right.
    >
    >The Garmin's data showed it to be about a mile wrong so I'm thinking,
    >either;
    >
    >1. The Garmin is wrong (Doubtful)


    I expect that the Garmin datum may not be set to the same as that used
    by the co-ordinates source (Streetmap in this case). Streetmap's
    co-ordinates will either be in OSGB36 or WGS-84. The Garmin datum is set
    under the Setup, Units, Map Datum field. Garmin call OSGB36 Ord Svy GB.

    >2. Streetmap.com's data may be incorrect (possible but not sure).
    >
    >On reaching the location I took an actual reading from the Garmin and
    >they are as follows;
    >
    >Incorrect N51:20:10, E0:0:16
    >Actual N21:21:00, E0:0:33


    The other thing to check is that the format is the same. In the example
    above it looks like you are working in degrees, minutes and seconds. Are
    you sure that

    (a) streetmap is giving you the co-ordinates in that and not
    degrees and decimal minutes for instance

    and

    (b) the Garmin's Position Format is set to hddd mm ss.s



    --

    Dominic Sexton
     
  5. Geoff,

    I can think of 2 potential problems :-

    1. The lat, lon returned from Streetmap is probably the postcode
    'centroid' - this is the point at the centre of the postcode polygon.
    Most postcodes cover a range of addresses and so are a polygon
    containing an area. This is approximated to a centroid on Streetmap in
    order to simplify it. You were probably directed by the eTrex to the
    centroid rather than a speicifc address. The actual centroid is probably
    in the middle of someone's back garden.

    2. You may have the datum set incorrectly. Streetmap will give you lat
    and lon in the OSGB1936 datum whereas your etrex will natively use the
    WGS84 datum unless you have set it otherwise. The difference between the
    two is about 400m.

    Regards

    Andrew


    --
    Andrew Whaley, author of :-

    Trailgauge - Shareware 3D GPS Mapping Software
    Free Download from http://www.trailgauge.com
     
  6. Al

    Al Guest

    "Andrew Whaley" <[email protected]> wrote in message
    news:[email protected].com...
    > 2. You may have the datum set incorrectly. Streetmap will give you lat and
    > lon in the OSGB1936 datum whereas your etrex will natively use the WGS84
    > datum unless you have set it otherwise. The difference between the two is
    > about 400m.
    >


    Andrew are you sure that this is the datum streetmap give because if you use
    this page it specifies that lat long is in WGS84 in the results
    http://www.streetmap.co.uk/gridconvert.html

    Rgds Alan
     
  7. Phil Cook

    Phil Cook Guest

    On Mon, 10 Jan 2005 18:14:25 +0000, Geoff Lane wrote:

    >Are there any other sites other than www.streetmap.com that will give
    >a location if you enter a map co-ordinate?
    >
    >Reason I ask is that I have a Garmin Etrex and I have often used it to
    >get to a location by entering the postcode on Streemap.com which then
    >gives me numerous map references - last week I used it to go to a
    >location near Croydon and when I neared my destination it was advising
    >a left turn where in fact I needed to turn right.
    >
    >The Garmin's data showed it to be about a mile wrong so I'm thinking,
    >either;
    >
    >1. The Garmin is wrong (Doubtful)
    >2. Streetmap.com's data may be incorrect (possible but not sure).


    Quite likely. Postcodes can be as precise as one building or as vauge
    as the whole of a long street.

    What I would do is find the street in streetmap and then go to the OS
    1:50k mapping on streetmap (click the arrows between the + and -) and
    check or read off the Grid Reference for the address and then enter
    that into your GPS as a waypoint. In this case your GPS should be set
    to OSGB position format and Ord Srvy map datum.
    --
    Phil Cook looking north over the park to the "Westminster Gasworks"
     
  8. Bitstring <[email protected]>, from the
    wonderful person Phil Cook <[email protected]> said
    >On Mon, 10 Jan 2005 18:14:25 +0000, Geoff Lane wrote:
    >
    >>Are there any other sites other than www.streetmap.com that will give
    >>a location if you enter a map co-ordinate?
    >>
    >>Reason I ask is that I have a Garmin Etrex and I have often used it to
    >>get to a location by entering the postcode on Streemap.com which then
    >>gives me numerous map references - last week I used it to go to a
    >>location near Croydon and when I neared my destination it was advising
    >>a left turn where in fact I needed to turn right.
    >>
    >>The Garmin's data showed it to be about a mile wrong so I'm thinking,
    >>either;
    >>
    >>1. The Garmin is wrong (Doubtful)
    >>2. Streetmap.com's data may be incorrect (possible but not sure).

    >
    >Quite likely. Postcodes can be as precise as one building or as vauge
    >as the whole of a long street.


    Or, out here, as vague as most of a village - that's about 2-3 square
    miles by my calculations. Steetmap gives a one point estimate for the
    post code, after that you're on your own. Any day now the PO will decide
    to use 12 digit OS references instead. 8>.

    --
    GSV Three Minds in a Can
    Outgoing Msgs are Turing Tested,and indistinguishable from human typing.
     
  9. Phil Cook <[email protected]> writes:

    >On Mon, 10 Jan 2005 18:14:25 +0000, Geoff Lane wrote:


    >>Are there any other sites other than www.streetmap.com that will give
    >>a location if you enter a map co-ordinate?


    >>Reason I ask is that I have a Garmin Etrex and I have often used it to
    >>get to a location by entering the postcode on Streemap.com which then
    >>gives me numerous map references - last week I used it to go to a
    >>location near Croydon and when I neared my destination it was advising
    >>a left turn where in fact I needed to turn right.
    >>
    >>The Garmin's data showed it to be about a mile wrong so I'm thinking,
    >>either;
    >>
    >>1. The Garmin is wrong (Doubtful)
    >>2. Streetmap.com's data may be incorrect (possible but not sure).


    >Quite likely. Postcodes can be as precise as one building or as vauge
    >as the whole of a long street.


    >What I would do is find the street in streetmap and then go to the OS
    >1:50k mapping on streetmap (click the arrows between the + and -) and
    >check or read off the Grid Reference for the address and then enter
    >that into your GPS as a waypoint. In this case your GPS should be set
    >to OSGB position format and Ord Srvy map datum.


    But even that can sometimes be wrong, due to errors somewhere. I've
    been finding addresses or post codes for some time in Streetmap, then
    refining it by clicking on the spot, and using the OSGB map ref as a
    waypoint. But a while back in London it reliably gave the wrong ref by
    about 400m, which had me going round on foot in circles until I
    stopped believing the GPS and going back to map navigation.

    I checked it out carefully afterwards, and there's a section of
    the map where all refs are displaced by the same amount NE.

    The address is Grafton Terrace, London, NW5 4HY, if anyone wants to
    check it out. Streetmap gives map ref 527750 185250 for this, but
    using that grid ref as input jumps the pointer to Duboyne Rd, about 400m
    NE. Clicking on these two different points of the map gives the
    *same* grid ref.

    I wrote to Streetmap two months ago with those details. They
    acknowledged my email immediately "will look into it, you'll hear from
    us later", but nothing since. I just checked to see if they've fixed
    it -- they haven't.

    So I've added an extra routine in my use of Streetmap to check against
    this. After clicking on the point to give me the OSGB ref, I simply
    then click on "SEARCH" again so that it searches for the grid ref it
    already supplied. The pointer shouldn't move. If it jumps the pointer
    badly that is the symptom of this particular error.

    Apart from that one time, I've found this be a very useful and
    reliable way of getting to places. The nominal accuracy is 50m plus
    mapping errors. When moving within the grid of city streets that
    almost always means you're in the right street and the place is
    visible by the time you reach the waypoint.
    --
    Chris Malcolm [email protected] +44 (0)131 651 3445 DoD #205
    IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
    [http://www.dai.ed.ac.uk/homes/cam/]
     
  10. In message <[email protected]>, Geoff Lane
    <[email protected]> writes
    >Are there any other sites other than www.streetmap.com that will give
    >a location if you enter a map co-ordinate?
    >
    >Reason I ask is that I have a Garmin Etrex and I have often used it to
    >get to a location by entering the postcode on Streemap.com which then
    >gives me numerous map references - last week I used it to go to a
    >location near Croydon and when I neared my destination it was advising
    >a left turn where in fact I needed to turn right.
    >
    >The Garmin's data showed it to be about a mile wrong so I'm thinking,
    >either;
    >
    >1. The Garmin is wrong (Doubtful)
    >2. Streetmap.com's data may be incorrect (possible but not sure).
    >
    >On reaching the location I took an actual reading from the Garmin and
    >they are as follows;
    >
    >Incorrect N51:20:10, E0:0:16
    >Actual N21:21:00, E0:0:33
    >
    >The incoorrect reference shows on streetmap.com as in Layhams Road but
    >my Garmin was suggesting West of Henrys Drive into Fairchilds Avenue.
    >
    >I appreciate there are inaccuracies in GPS but this was telling me a
    >deffinite left was needed whereas it was actually a right turn.
    >
    >Any comments or suggestions welcomed.



    I have postcode data available, though only to 100m accuracy. It all
    comes from the same source originally, though having checked Chris's
    example from elsewhere in the thread my figures differ from streetmap in
    the last digit, even after maximum allowance for rounding. This makes a
    minimum discrepancy of 223m (100m Easting and 200m Northing). My
    figures are SE of streetmap, but unfortunately Chris says the deviation
    is NE.

    If you tell me the postcode you entered I can let you have my figures
    for comparison and will also might be able to tell you whether the
    postcode has any particular features that might explain the deviation.


    --
    Michael Farthing
    Aardvark Ltd
     
  11. GSV Three Minds in a Can wrote:

    > Any day now the PO will decide
    > to use 12 digit OS references instead. 8>.


    No we wont.
    See Andrew Whaley's post.

    Anyway, we're Royal Mail ;-P

    Chris
     
  12. Geoff Lane

    Geoff Lane Guest

    On Mon, 10 Jan 2005 18:57:19 +0000, Dominic Sexton
    <{d-sep03}@dscs.demon.co.uk> wrote:

    >I expect that the Garmin datum may not be set to the same as that used
    >by the co-ordinates source (Streetmap in this case). Streetmap's
    >co-ordinates will either be in OSGB36 or WGS-84. The Garmin datum is set
    >under the Setup, Units, Map Datum field. Garmin call OSGB36 Ord Svy GB.


    Never even looked at that, my Garmin is set to default which I've just
    looked at and is WGS-84

    >The other thing to check is that the format is the same. In the example
    >above it looks like you are working in degrees, minutes and seconds. Are
    >you sure that


    Yes, I sussed the difference and am intentionally using degrees,
    minutes and seconds.

    Geoff Lane
     
  13. Geoff Lane

    Geoff Lane Guest

    On Mon, 10 Jan 2005 20:08:04 +0000 (UTC), Andrew Whaley
    <[email protected]> wrote:


    >1. The lat, lon returned from Streetmap is probably the postcode
    >'centroid' - this is the point at the centre of the postcode polygon.
    >Most postcodes cover a range of addresses and so are a polygon
    >containing an area. This is approximated to a centroid on Streetmap in
    >order to simplify it. You were probably directed by the eTrex to the
    >centroid rather than a speicifc address. The actual centroid is probably
    >in the middle of someone's back garden.


    No, I've rechecked it by entering the long/lat into streetmap.com and
    it shows on the map a different location to where it directed me; not
    too far out but about half a mile al least.

    Geoff Lane
     
  14. Geoff Lane

    Geoff Lane Guest

    On Mon, 10 Jan 2005 21:51:04 +0000, [email protected] wrote:

    >I agree with others, your datum in the garmin is still osgb36 when
    >these mapping programs all seem to use wgs84 and utm.


    I've checked my Garmin and it shows WGS84

    Geoff Lane
     
  15. Geoff Lane

    Geoff Lane Guest

    On Mon, 10 Jan 2005 23:54:23 +0000, Phil Cook
    <[email protected]> wrote:


    >What I would do is find the street in streetmap and then go to the OS
    >1:50k mapping on streetmap (click the arrows between the + and -) and
    >check or read off the Grid Reference for the address and then enter
    >that into your GPS as a waypoint. In this case your GPS should be set
    >to OSGB position format and Ord Srvy map datum.


    I'll give this a try - I am returning to the location next week so
    will have another go.

    I live in NW London and this location is in the Kent area.

    Geoff Lane
     
  16. Phil Cook

    Phil Cook Guest

    On Thu, 13 Jan 2005 19:16:20 +0000, Dave Fawthrop wrote:

    >On Wed, 12 Jan 2005 18:17:57 +0000, Geoff Lane
    ><[email protected]> wrote:
    >
    >| On Mon, 10 Jan 2005 21:51:04 +0000, [email protected] wrote:
    >|
    >| >I agree with others, your datum in the garmin is still osgb36 when
    >| >these mapping programs all seem to use wgs84 and utm.
    >|
    >| I've checked my Garmin and it shows WGS84
    >
    >Then reset it to National grid for use in the uk.


    However then make sure you feed it positions in OSGB datum. The
    easiest way to do that is to feed it positions in Grid Refference form
    rather than Lat Long. If you do feed it Lat Long you need to be sure
    which Lat Long the position is published in.

    On Streetmap Lat Long is given as WGS84 (in two formats D:M:S and
    decimal Degrees) so if you were entering that you would hve to set
    your GPS to WGS84. However to enter the Grid Reference which streetmap
    call LR (Landranger Reference) you will have to have yiour GPS set to
    OSGB and add in the zeros to convert the six figure reference to a ten
    figure one:

    AB 123456 becomes AB 1230045600

    >I had my Garmin set to WGS84 and it was about a kilometer out from National
    >grid, OS Explorer maps. I changed it to whatever garmin call the
    >National grid and it is now near enough spot on against the National Grid.
    >
    >All maps I have ever seen in the UK use the national grid, even though they
    >quite often do not state it explicitly. There was a thread about pre war
    >maps on urw recently.


    I have an AA road atlas knocking about in which I pencilled in the
    National Grid letters and numbers on the 10km grid on the page
    margins.
    --
    Phil Cook looking north over the park to the "Westminster Gasworks"
     
  17. Geoff Lane

    Geoff Lane Guest

    On Wed, 12 Jan 2005 21:02:15 +0000, Phil Cook
    <[email protected]> wrote:


    >>| >I agree with others, your datum in the garmin is still osgb36 when
    >>| >these mapping programs all seem to use wgs84 and utm.
    >>|
    >>| I've checked my Garmin and it shows WGS84
    >>
    >>Then reset it to National grid for use in the uk.

    >
    >However then make sure you feed it positions in OSGB datum. The
    >easiest way to do that is to feed it positions in Grid Refference form
    >rather than Lat Long. If you do feed it Lat Long you need to be sure
    >which Lat Long the position is published in.


    If I reset my Garmin will the waypoints I already have in the Garmin
    still be valid?

    This mapping theory is quite complicated but fascinating.

    Geoff Lane
     
  18. Bitstring <[email protected]>, from the
    wonderful person Geoff Lane <[email protected]> said
    >On Wed, 12 Jan 2005 21:02:15 +0000, Phil Cook
    ><[email protected]> wrote:
    >
    >
    >>>| >I agree with others, your datum in the garmin is still osgb36 when
    >>>| >these mapping programs all seem to use wgs84 and utm.
    >>>|
    >>>| I've checked my Garmin and it shows WGS84
    >>>
    >>>Then reset it to National grid for use in the uk.

    >>
    >>However then make sure you feed it positions in OSGB datum. The
    >>easiest way to do that is to feed it positions in Grid Refference form
    >>rather than Lat Long. If you do feed it Lat Long you need to be sure
    >>which Lat Long the position is published in.

    >
    >If I reset


    if you mean 'change the datum/format to WGS84' then yes.. if you mean
    'reset', as in 'hard reset', you lose everything, but it's pretty hard
    to do that (not even in the manual IIRC)

    >my Garmin will the waypoints I already have in the Garmin
    >still be valid?


    Yes, internally they all store locations as WGS84 .. it's only for
    input/output purposes that they are willing to try to translate them
    into the 1001 different sorts of garbage in use around the world.

    Another wrinkle that I just tripped over and had forgotten about - OS
    grid references are always truncated, never rounded. Not a problem if
    you use 10 digit references, but if not then SO12399 45699 turns into a
    6 figure reference SO123456 .. about 1.4Km out from the first one.

    --
    GSV Three Minds in a Can
    Outgoing Msgs are Turing Tested,and indistinguishable from human typing.
     
  19. Phil Cook

    Phil Cook Guest

    On Thu, 13 Jan 2005 01:42:46 +0000, GSV Three Minds in a Can wrote:

    >Another wrinkle that I just tripped over and had forgotten about - OS
    >grid references are always truncated, never rounded. Not a problem if
    >you use 10 digit references, but if not then SO12399 45699 turns into a
    >6 figure reference SO123456 .. about 1.4Km out from the first one.


    You're an order of magnitude out there! It's only about 140m from
    SO123456 to SO1239945699
    --
    Phil Cook looking north over the park to the "Westminster Gasworks"
     
  20. In article <[email protected]>, GSV Three Minds in a Can
    <[email protected]> writes
    >Bitstring <[email protected]>, from the
    >wonderful person Geoff Lane <[email protected]> said
    >>On Wed, 12 Jan 2005 21:02:15 +0000, Phil Cook
    >><[email protected]> wrote:
    >>
    >>
    >>>>| >I agree with others, your datum in the garmin is still osgb36 when
    >>>>| >these mapping programs all seem to use wgs84 and utm.
    >>>>|
    >>>>| I've checked my Garmin and it shows WGS84
    >>>>
    >>>>Then reset it to National grid for use in the uk.
    >>>
    >>>However then make sure you feed it positions in OSGB datum. The
    >>>easiest way to do that is to feed it positions in Grid Refference form
    >>>rather than Lat Long. If you do feed it Lat Long you need to be sure
    >>>which Lat Long the position is published in.

    >>
    >>If I reset

    >
    >if you mean 'change the datum/format to WGS84' then yes.. if you mean
    >'reset', as in 'hard reset', you lose everything, but it's pretty hard
    >to do that (not even in the manual IIRC)
    >
    >>my Garmin will the waypoints I already have in the Garmin
    >>still be valid?

    >
    >Yes, internally they all store locations as WGS84


    Well if you put a waypoint in manually with the receiver set to OSGB36
    and then set the receiver to WGS-84 the co-ordinates would change (even
    though the actual point is still in the same place). If the co-ordinates
    were originally in WGS-84 but were entered manually with the receiver
    set to OSGB36 the point would still be in the wrong place :-(

    > .. it's only for input/output purposes that they are willing to try to
    >translate them into the 1001 different sorts of garbage in use around
    >the world.
    >
    >Another wrinkle that I just tripped over and had forgotten about - OS
    >grid references are always truncated, never rounded.


    Depends on who supplies them - we have argued the point here more than
    once a while back.

    > Not a problem if you use 10 digit references, but if not then SO12399
    >45699 turns into a 6 figure reference SO123456 .. about 1.4Km out from
    >the first one.


    ~141m actually for a 6 fig ref.

    --

    Dominic Sexton
     
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