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Why the grid north doesn't agree with true north on maps

  1. May 25, 2016 #1
    when I look at large scale map such as USGS top maps, i noticed that maps' grid are not aligned with the map frame. looking at the declination diagram at the bottom of the map, there's a difference between true north and GN (Grid north)

    can someone explain to me why the map grid doesn't align with True north?
     
  2. jcsd
  3. May 25, 2016 #2

    davenn

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  4. May 25, 2016 #3

    Borg

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    True north and magnetic north aren't located in the same place and differ by a large distance. Depending on how they align with respect to your location, there can be a significant angular difference. In Europe, the angle is fairly small. However, the farther north you go in North America, the difference becomes quite large.
     
  5. May 25, 2016 #4
    thanks for the reply, i understand the difference bet. true north and magnetic north. the question i have is why the difference between True north and Grid north. please see attached picture USGS top map Declination diagram, there is a difference of 1 deg 34 mins between true north and grid north, can anyone explain this difference?
     

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  6. May 25, 2016 #5

    SteamKing

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    It has to do with how the surface of an oblate spheroid like the earth is projected onto a two-dimensional flat surface while minimizing distortion of the geographic features from the three-dimensional surface.

    Part of the UTM projection is explained in this USGS article:

    https://pubs.usgs.gov/fs/2001/0077/report.pdf

    When detailed maps of a region are produced, a central meridian is chosen which usually runs thru the center of the region being depicted. The further away one goes from this central meridian, the more distortion accumulates on the two-dimensional map, so large areas, like the continental United States, are usually split up so that several different reference meridians are used. UTM is a global projection, but the U.S. also has a separate projection system called State Plane, and the 48 continental states plus Alaska and Hawaii are divided into one or more regions for the purpose of making 2-D maps of these territories. In each system, State Plane or UTM, the 2-D map is designed so that features can be located with a pair of X-Y coordinates, with X being measured east and west from the central meridian while Y is measured north and south from a reference latitude. UTM measures X-Y in meters while State Plane uses feet.

    The science of making maps of the earth's surface is called geodesy, and the USGS has several publications which describe map making and map projections in more detail.
     
  7. May 26, 2016 #6
    thanks for the explanation.

    I calibrated a USGS topo maps with 2-point diagonal calibration and use on my phone for tracking. then I realized that the map are off when I'm tracking with GPS. when I looked closely at the map, I found out the grids are not aligned with map edge, the map frame is oriented to True north, but the grid north is off by a small degree, this would require at least 3-points calibration to be used accurately with GPS. so I calibrated with 4-points and it turned out to be accurate.
     
  8. Jun 15, 2016 #7
    GN stands for Geographic North, not grid north. GN is the point around which the earth rotates. It doesn't line up precisely with Polaris, the North Star. The little diagram on the map shows the variance between GN and the North Star.

    The grids on the map are usually Range and Township lines. The grid we use for surveying land.
     
  9. Jun 15, 2016 #8

    Borg

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    That is not correct. GN refers to Grid North.

    Declination_diagram.png

    http://therucksack.tripod.com/MiBSAR/LandNav/Datums/1952USGS1-62DatumLegend.jpg [Broken]
     
    Last edited by a moderator: May 8, 2017
  10. Jun 15, 2016 #9
    A matter of semantics. Here, the grid they speak of is the UTM grid. Which is the same as the longitude and latitude lines. Which meet at Geographic North on the globe. We're saying the same thing.
     
  11. Jun 16, 2016 #10

    Borg

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    No we're not. Geographic coordinates are very different from grid coordinates. The star in the legend represents geographic north and the GN angle represents the angular difference between geographic and grid coordinates at the center of the sheet. This particular map shows two grids (UTM in blue and 1927 North American).

    grid.jpg

    Also, take a look at this map of an area in northern Russia. You will see that the geographic coordinates actually curve on the map where the grid coordinates do not. For maps at this latitude the angular difference between geographic and grid coordinates varies too much across the sheet to give a single value from the center.
     
    Last edited: Jun 16, 2016
  12. Jun 16, 2016 #11

    Borg

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    Correction: The line from the 1927 North American datum should be pointed to the 320000 FEET tick between the two blue circles.
     
  13. Jun 16, 2016 #12

    D H

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    This is very wrong, and Borg is correct. GN stands for grid north, not geographic north. Look at different topographic maps. Every topographic map has three different concepts of "north". True north, which points toward the Earth's rotation axis; grid north, which is parallel to the vertical edge paper on which the map is printed; and magnetic north, which is the direction in which a compass points. The question is, why is this done?


    The basic problem is that maps are flat while the Earth is more or less spherical. There is no perfect representation of the surface of the Earth on a flat piece of paper. Every projection involves some sort of compromise. Lines of latitude and longitude are orthogonal to one another in a Mercator projection. This projection does a nice job near the equator, but it is rather lousy more than a few degrees from the equator.

    Topographic maps instead typically use a transverse Mercator projection. A transverse Mercator projection generalizes the concept of Mercator projection to some great circle other than the equator. The universal transverse Mercator maps used in topographic maps are a bit of a misnomer. The problem with any Mercator projection is that they are not "universal." A Mercator projection is rather lousy in places far from the great circle used as the basis for the projection.

    The solution used in UTM projections is to use sixty different transverse Mercator projections, each centered on a line of latitude rather than the equator. The sixty different projections correspond to 6 degree wide slices of an orange. This reduces the distortions that result from using a global Mercator projection, but at the cost of having lines of latitude and longitude being curves rather than lines.

    Finally, to answer the question, true north typically is not parallel to an edge of the paper on which a topographic map is printed.
     
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