About GPS Settings & Datum

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0 height="100%"><tr> <td valign="top" width=200 BGCOLOR="#000000" TEXT="#F7F7FF"> <div> <IMG SRC="17786860.jpg" border=0 width="134" height="134" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Unfortunately, the Earth is not a perfect sphere: it's sort of squashed into what is technically referred to as an 'oblate spheroid'. </div> <div> This is caused by rotational effects and creates havoc when you're trying to survey parcels of land down to a reasonable accuracy.</div> <div> Whenever we want to represent our local area on a map, we insist that the map is divided up into a square grid pattern, but when we try to place a square pattern over the globe, we have a small problem:</div> <div> <IMG SRC="0f4a1490.jpg" border=0 width="161" height="73" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Nothing quite fits. </div> <div> It's here that we use various tricks to make our maps at least appear to be of a flat earth.</div> <div> NZ Geodetic Datum of 1949 provides our current method of mapping our flat topographic maps onto a theoretical model of the Earth</div> <bR> </td> <td valign="top" BGCOLOR="#99CCFF" TEXT="#080000"> <div> <FONT SIZE="5" COLOR="#003333" FACE="Arial" ><B>4WD North Island: 102 Off Road Adventures</B></FONT>     |     <A HREF="index.htm" TARGET="_top" TITLE="4WD North Island: 102 Off Road Adventure"><U>home</U></A></div> <div> <B>About GPS Settings & Datum</B><B>   |   </B><A HREF="id30.htm" TARGET="_top" TITLE="The Snail Trails"><U>The Snail Trails</U></A><B>   |   </B><A HREF="id33.htm" TARGET="_top" TITLE="Links"><U>Links</U></A></div> <div> About GPS Settings & Datum</div> <bR> <DIV ALIGN="LEFT"></DIV> <div> <B><U>Let's stick with NZ Geodetic Datum 1949</U></B><FONT SIZE="2" COLOR="#000000" FACE="Arial" > (for now)</FONT></div> <bR> <div> All GPS information used here is based upon the 1949 Geodetic Datum, which is a mathematical model approximating the shape of the world, at least from a New Zealand perspective. When you consider the difficulties a pre-satellite society must have had actually calculating the curvature of the planet, you must agree they did pretty well in 1949.</div> <bR> <div> We now know through modern satellite positioning systems and exceptionally sophisticated surveys that the true model is slightly different. This was generally adopted as the World Geodetic Standard of 1984 (sometimes called WGS84), although New Zealand never actually adopted this standard, preferring instead to retain its own standard now commonly known as the NZ Geodetic Datum 1949, or NZGD49.</div> <bR> <div> Plans are afoot for New Zealand to adopt a new and more accurate model, the World Geodetic Standard 2000 (or WGS2000), but while that has been more or less finalised, it has not yet gained widespread acceptance. When it does happen, all grid coordinates in New Zealand will shift by about 200 metres. But until the new standard is widely accepted, we will stick with what everyone knows — NZGD49. This is, after all, the basis of all the 260 Series Topographic Maps.</div> <bR> <div> For those interested in reading further, this article is taken from the Land Information New Zealand Fact Sheet, September 1998:</div> <div> <I>New Zealand Geodetic Datum 2000 (NZGD2000).</I></div> <div> <I>This fact sheet explains New Zealand Geodetic Datum 2000, the form of this new datum, how it will impact on users, and future developments. </I></div> <bR> <div> <I>New Zealand Geodetic Datum 2000 Approved</I></div> <div> <I>Land Information New Zealand is implementing a new geodetic datum, New Zealand Geodetic Datum 2000 (NZGD2000), to replace New Zealand Geodetic Datum 1949 (NZGD49). The new datum will provide an accurate infrastructure for spatial referencing in New Zealand as we move into the information age of the 21st century. </I></div> <div> <I>The decision was made following discussion and consultation with users, and an evaluation of options against business drivers. The primary business driver is the need to support the survey and title automation project Landonline. The new datum will become available in 1999.</I></div> <div> <I>The characteristics of NZGD2000 are: </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The datum will have a geocentric origin which is compatible with the Global Positioning System (Note: the centre of the NZGD2000 ellipsoid will coincide with the centre of mass of the earth. The centre of the ellipsoid used to define NZGD49 is displaced approximately 200m from the earth's centre of mass.) </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">In line with recommendations by the International Association of Geodesy, the datum will be based on, and aligned with, the International Terrestrial Reference System (ITRS) and the ellipsoid associated with this datum will be the Geodetic Reference System 1980 (GRS80) ellipsoid.</I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The relationship between the new datum and the ITRS will be realised through the International Terrestrial Reference Frame 1996 (1TRF96) coordinates, specified at epoch 2000.0 (a reference date of 1 January 2000). All points coordinated in terms of the new datum will have coordinates defined in terms of this epoch (Note: the ITRF96 and World Geodetic System 1984 (WGS84) reference frames agree with each other to within approximately 10cm). </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The generalised motion of points in New Zealand with respect to the ITRS will be modelled: </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">to ensure that Land Information NZ can generate epoch 2000.0 coordinates from observations made at other times </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">to allow other specialised users to generate up-to-date coordinates for times other than the reference epoch. </I></div> <div> <I>What is a datum?</I></div> <div> <I>A datum is a mathematically defined reference surface, approximating the shape of the earth, that enables calculations such as position and area to be carried out in a consistent and accurate manner. The datum is physically represented by a framework of ground monuments (i.e. trig stations) whose positions have been accurately measured and calculated on this reference surface. </I></div> <bR> <div> <I>New Zealand Geodetic Datum 1949</I></div> <div> <I>New Zealand Geodetic Datum 1949 (NZGD49) was introduced to meet the needs for an integrated spatial reference system and complete topographical and cadastral mapping database coverage of New Zealand. Since its completion in 1949 it has played a significant role in the subsequent development of New Zealand. Because of the long-term nature of land use, spatial systems need to be robust and capable of supporting future applications and growth. </I></div> <div> <I>While NZGD49 has served New Zealand's needs well over the last 50 years, it cannot now match the accuracy of new technologies such as the Global Positioning System (GPS) or unified high accuracy GIS applications over large areas. There is a need to account for the effects of ground deformation to maintain an accurate survey infrastructure. This, and the development of an accurate integrated automated survey and title system, Landonline, means there is a requirement for a new geodetic network and datum, able to serve New Zealand's needs for the foreseeable future. </I></div> <bR> <div> <I>Limitations of NZGD49</I></div> <div> <I>The limitations with the current datum are that:</I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">Regional distortions are present in the network of up to 5m due to the lower precision of the survey observations and techniques used to define the datum and the effects of crustal deformation since it was surveyed. </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The lower-order breakdown has been built up and defined in a piecemeal fashion resulting in localised distortions. </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">It is incompatible with global geodetic reference systems. </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">It is of limited spatial coverage, covering only the land mass of New Zealand. </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">It does not define a vertical datum. </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">It is static — the coordinates of the first-order stations that define NZGD49 are held fixed, irrespective of subsequent movements. </I></div> <bR> <div> <I>Advantages of NZGD 2000</I></div> <div> <I>The advantages of the new datum are that: </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">It will be a three dimensional datum compatible with international geodetic systems, such as the International Terrestrial Reference System (ITRS) and the World Geodetic System 1984 (WGS84) </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The accuracy of the datum will support modern survey techniques and provide a network of modern, accessible survey marks </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The effects of slow crustal deformation (plate tectonics) amounting to about 5cm/year, will be managed through the use of a velocity model. The velocity model will enable Land Information NZ to generate NZGD2000 coordinates from observations made at times other than the datum reference epoch. This will allow specialised users to generate up to date coordinates for times other than the reference epoch </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">The NZGD2000 coordinates of marks will change where new observations provide more accurate values, or when marks are affected by unpredictable deformation such as earthquakes. </I></div> <bR> <div> <I>The Impact of NZGD2000 </I></div> <div> <I>Coordinates will change</I></div> <div> <I>The NZGD2000 coordinates (latitude and longitude) of a point will be different to its NZGD49 coordinates. This difference is mainly because NZGD2000 is a geocentric datum whereas NZGD49 is not. The effect of moving to a geocentric datum is as if New Zealand had shifted approximately 190m north and l0m east. Transformations will be developed to enable users to transform coordinates between NZGD49 and NZGD2000. </I></div> <bR> <div> <I>Timeframe for implementation</I></div> <div> <I>The initial surveying and calculation of the NZGD2000 network and datum will be completed by 1 January 1999. At this time further detailed information about the datum will be available, including transformation parameters relating NZGD49 to NZGD2000. </I></div> <div> <I>Implementation will commence as the Landonline project introduces automated systems from mid 1999.</I></div> <div> <I> </I></div> <div> <I>Further projects </I></div> <div> <I>A number of other projects will be implemented as a result of the new datum decision. These include: </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">assessment and development of new map projections </I></div> <div> <I><IMG BORDER="0" SRC="Symb075c00b700c800000000.gif" HEIGHT="13" WIDTH="24" ALIGN="bottom" HSPACE="0" VSPACE="0">development of transformations between the new datum and other data/projections.</I></div> <bR> <div> <I>Further information</I></div> <div> <I>Details of the rationale behind the decision for the new datum is available on the Internet at:</I></div> <div> <A HREF="http://www.linz.govt.nz" TARGET="_top" TITLE="http://www.linz.govt.nz"><U><U>http://www.linz.govt.nz</U></U></A><FONT SIZE="2" COLOR="#000000" FACE="Arial" ><I> (search for OSG Technical Report 2.1).</I></FONT></div> <div> <I>A copy of the New Zealand Geodetic Strategic Business Plan detailing further development of the geodetic system in New Zealand is available on the Internet at:</I></div> <div> <A HREF="http://www.linz.govt.nz" TARGET="_top" TITLE="http://www.linz.govt.nz"><U><U>http://www.linz.govt.nz</U></U></A><FONT SIZE="2" COLOR="#000000" FACE="Arial" ><I> (search for OSG Technical Report 3).</I></FONT></div> <bR> <div> <I>NOTE: TO AVOID CONFUSION, USERS OF GEODETIC COORDINATES</I></div> <div> <I>MUST BE AWARE OF WHICH DATUM THE COORDINATES ARE IN TERMS OF </I></div> <bR> <div> <I>As the datum is developed, further information will become available from:</I></div> <div> <I>Surveyor-General</I></div> <div> <I>Land Information NZ</I></div> <div> <I>P0 Box 5501</I></div> <div> <I>WELLINGTON</I></div> <div> <I>NEW ZEALAND</I></div> <bR> </td> </tr></table></body>