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Differentiation between Global and Local Datum from Different aspect
- 1. Differentiation between Global and Local Datum from Different aspect Student Name: Copyright Class: 3rd Stage Course Title: Digital Mapping Department: Geomatics (Surveying) Engineering College of Engineering Salahaddin University-Erbil Academic Year 2019-2020
- 2. 1 ABSTRACT Spatial professionals are required to deal with an increasingly wide range of positioning information obtained from various sources including terrestrial surveying, Global Navigation Satellite System (GNSS) observations and online GNSS processing services. These positions refer to a multitude of local, national and global datums. A clear understanding of the different coordinate reference systems and datums in use today and the appropriate transformations between these is therefore essential to ensure rigorous consideration of reference frame variations in order to produce high-quality outcomes in spatial data analysis tasks. The aim of creating this report is showing the importance of digital mapping, the digital mapping is global datum and local datum. The report generally consist of the definition of the global datum and local datum, and its difference between both of them changes by its place and its location . They complete each other it means that when global datum exists the local datum exist too and the vice vecra . And I discussed about the advantages and disadvantages between both of datums.
- 3. 2 Table of content Subject page ABSTRACT................................................................................................1 Table of content ..........................................................................................2 Introduction.................................................................................................3 Background & Review................................................................................5 Methods.......................................................................................................9 Theory / Design.........................................................................................12 Conclusion ................................................................................................17 Reference ..................................................................................................18
- 4. 3 Introduction Digital mapping (also called digital cartography) is the process by which a collection of data is compiled and formatted into a virtual image. The primary function of this technology is to produce maps that give accurate representations of a particular area, detailing major road arteries and other points of interest. Datums are used in digital mapping, navigation, and surveying by cartographers and satellite navigation systems to translate positions indicated on maps (paper or digital) to their real position on Earth. Each starts with an ellipsoid (stretched sphere), and then defines latitude, longitude and altitude coordinates. One or more locations on the Earth's surface are chosen as anchor "base- points". The difference in co-ordinates between datums is commonly referred to as datum shift. The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of meters (or several kilometers for some remote islands). Datums are the basis for all geodetic survey work. A geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps. In this way, datums act similar to starting points when you give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a crossroads or a building address. There are two main datums in the United States. Horizontal datums measure positions (latitude and longitude) on the surface of the Earth, while vertical datums are used to measure land elevations and water depths.
- 5. 4 The datum represents a vital but frequently overlooked part of a map or digital map data. The datum consists of a starting point and orientation, and an assumed shape of the earth (ellipsoid). For historical reasons a wide variety of local datums have been used in mapping. NIMA's TR 8350.2 lists about 200 datums. Some of the reasons for the multitude of datums include historical pride, but different datums best fit different parts of the world, and in the 19th century it was not easy to survey islands or colonial outposts relative to a known location and to tie regions together. Although most coordinate systems now use longitude relative to Greenwich, that was not usually the true starting point for the projection; the coordinates of the true starting point (Meades Ranch, Kansas, for the historic US datum of NAD 1927) were merely adjusted to put Greenwich on the prime meridian after its universal designation in 1884. With the advent of space based geodesy, we can now develop true earth centered datums that provide a best fit ellipsoid valid over the entire globe. The most common global datum today is WGS84 (World Geodetic System), championed by NGA for world-wide mapping and the native reference for the GPS satellites. For all practical purposes WGS84 is the same as NAD83 (North American Datum 1983), and they merely used different formal definitions which lead to miniscule differences, although the continued drift of the continents complicates precise measurements. We recommend that you always use WGS84. All NGA data, most USGS data, and current TIGER data, come in that datum. The accepted European datum is ETRS89, which is also essentially the same as WGS84, but a number of the mapping agencies there still use their old national datums and projections.
- 6. 5 Background & Review in the past, positions with respect to horizontal and vertical datums have been determined independent of each other. In addition, horizontal datums were nongeocentric and were selected to best fit certain regions of the world (Figure 1). As such, those datums were commonly called local datums. More than 150 local datums have been used by different countries of the world. An example of the local datums is the North American datum of 1927 (NAD 27). With the advent of space geodetic positioning systems such as GPS, it is now possible to determine global 3-D geocentric datums. Figure 1 Geocentric and local datums.
- 7. 6 The foremost significance of geocentric datum is that a single Homogeneous geodetic datum can be recognized throughout the World. Thus, geocentric datum at national level can be Comprehended by linking its national reference frame to the Subset of geodetic stations from International GNSS Service (IGS) of the global frame from ITRF, hence implementing geocentric datum. The definite aims of modern geodesy is to present a highly stable geocentric reference datum for scientific purpose, such as precise orbit determination, monitoring sea level rise measuring plate tectonics and so on (Angermann et al., 2003). Every country tremendously requires geocentric datum with an appropriate procedure correlate with its regular update and maintenance. There are several alternatives towards the advancements of national geocentric datum such as updating a static datum regularly and implementation of the semi-dynamic and dynamic geocentric datums. A static datum is described as the datum that remains unchanged whereby the coordinates of geodetic stations are held fixed. While, a semi-dynamic datum (semi-kinematic datum) is described as the change of coordinates and it is implemented for calculating coordinates at different moments in times. On the other hand, dynamic datum (kinematic datum) is defined as a set of physical points attached to Earth’s surface. The points are fixed velocities and coordinates that continuously change over time due to crustal movements (Ronen and Even-tzur, 2017).
- 8. 7 There are really no advantages that a local geodetic datum have over a geocentric datum. The reason why local geodetic datums exist is because traditionally, when starting a survey, say for mapping purposes, the surveyor would determine latitude and longitude of his first point, or origin, astronomically. Direction would also be determined with astronomical observations and a long baseline would be physically measured in a certain direction. This baseline would then be used to establish a network of points using triangulation.. All this essentially yielded a local horizontal geodetic datum with a defined ellipsoidal surface and a definite orientation in space. The problems start when you want to connect two local geodetic datums with different origins and different orientations and possibly a different ellipsoidal surface. This leads to a re-computation of all the network observations from a common origin, which is an enormous task. Also, local geodetic networks are not connected to the centre of mass of the earth. With the advent of global navigation satellite systems (GNSS) and the subsequent development of space geodesy, it is necessary for a geodetic datum to be geocentric, because the satellites orbit around the centre of mass of the earth. These developments enabled scientists to establish a global geocentric geodetic datum, to which local geodetic datum stations could be connected using GNSS signals. Using enough stations on the local datum and obtaining global datum coordinates for them, the local horizontal geodetic datum can be mathematically transformed to be part of the global geocentric geodetic datum.
- 9. 8 So, horizontal local datums are becoming obsolete in the sense that they are not used for new networks or new geodetic surveys any more and nobody has to establish a local datum, on earth, ever again. That said, they still remain relevant for older maps and old surveys based on them, since those have to be transformed to the new datum before any new survey work, e.g. updating work, can be done in the area.
- 10. 9 Methods Spheroid/Ellipsoid-based surfaces use geometric calculations to define the local datum and a spheroid. It is important to note that it is impossible to transform between the geoid and Spheroid/Ellipsoid-based datums. This is because there are no common elements between the two reference systems and no site- specific gravity measurements in a spheroid/ellipsoid system so no mathematical transformation can be applied. There isn't one single ellipsoid that can best represent every part of the earth. Some ellipsoids are the best fit for the geoid for a certain region; however they can not accurately represent other parts of the world. Such ellipsoids are commonly used for a specific country or continent. Integration of data with non-matching coordinate systems from various regions would pose a challenge when working with different regionally fitting ellipsoids. An example of a regional best fitting ellipsoid is Airy 1830 used in Great Britain. There are also ellipsoids that provide an overall reasonable representation of the geoid as a whole; one popular example is the WGS84 ellipsoid employed by GPS systems. A local datum aligns its ellipsoid to closely fit the earth's surface in a particular area. A point on the surface of the ellipsoid is matched to a particular position on the surface of the earth (the origin point of the datum). The coordinates of this point are fixed and the corodinates of all other points are calculated from it. An Earth-Centered, or geocentric datum uses the Earth's center of mass as the origin point of the datum.
- 11. 10 Table1.Comparing Common U.S. Datums Figure 2: Global and Local geodetic datums and Geoid Such ellipsoids are conventionally defined as fitting the geoid in the region of interest, without them being necessarily geocentric or to have their axes constrained to a given orientation. A valuable contribution of GPS was to provide a Global Reference Frame, simplifying the mapping of the earth and unifying the diverse datums [Misra & Enge, 2001][1] . Figure 1 illustrates the concept of Local and Global datum, and its relation the geoid. Ellipsoid Earth- Centered or Local? Origin point of datum NAD27 Clarke 1866 Local Meade's Ranch, KS (coordinates determined using celestial measurements) NAD83 GRS80 Earth - Centered (Geocentric) Center of the Earth WGS84 WGS84 Earth - Centered (Geocentric) Center of the Earth
- 12. 11 In using digital data sets, you must be aware of the datum for several different things, and insure that they are correctly set: 1. The datum used to digitize the map. GIS software should correctly identify the datum for both USGS and NGA data sets and most other data sets that have metadata. World files and shape files will be notable exceptions because the formats do not include the projection and datum in their definitions, and the work-arouonds are not universal. 2. The datum used for local paper maps if you will use the maps to manipulate the digital data, or compare them to the on screen data. Paper maps and charts should show the datum in the legend or marginal information on the bottom of the map. Old U.S. maps that do not indicate the datum are almost certainly on NAD27 (or so old they might have not a datum). Datums affect both lat/long and UTM coordinates. 3. GPS units report coordinates by default on the WGS84 datum, but you can transform the coordinates to another datum. This might not be easy of a phone, which is replacing the more sophistiated and dedicated GPS units. If these datums are not correctly set, screen coordinates will not match up with coordinates on the paper map. If the two datums are correctly set, data set accuracy and resolution may affect how closely features on the screen (like hill tops or valleys) match up with their true coordinates. It is often better to select the high point in the data set rather than to rigidly rely on hilltop coordinates read from a map. GIS users who want the full power of the GIS to merge and combine different data sets must insure that they are on the same datum or have been shifted to lie on the same datum.
- 13. 12 Theory / Design A datum is a system which allows the location of latitudes and longitudes (and heights) to be identified onto the surface of the Earth - ie onto the surface of a ’round’ object. The basic mathematical/geometric principle which is used is that: » mathematically a ’round’ surface (a modified sphere) is created which represents the surface of the Earth » from here calculations are made to fit this mathematical model to the surface of the Earth - firstly the Equator, then North and South Poles and then lines of latitude and longitude. Refer to the section on the Earth’s coordinate system. Because there are different ways to fit the mathematical model to the surface of the Earth, there are many different datums. Also, in the modern digital era, techniques have vastly improved and many modern datum are very similar to each other. However, also in this modern digital era, people Figure 3
- 14. 13 like to know locations precisely so even a small difference may be significant A projection is a process which uses the latitude and longitude which has already been ‘drawn’ on the surface of the Earth using a datum, to then be ‘drawn’ onto a ‘flat piece of paper’ - called a map. See the section on Projections for more information about projection methodologies. The transformation of a local datum into a global (GPS) datum There is a high probability that within an engineering network we may find points which are coordinated in a global (e.g. GPS) reference frame. Conventionalyy such a global reference frame is located within the mass centre of the Earth. Its orthonormal base vectors refer to "Greenwich", "orthogonal to Greenwich in the equatorial plane of some reference epoch", "along the Earth rotation axis at some reference epoch". The IF'—frame is related to the IE-frame or the IF- frame ,respectively , by Figure 4
- 15. 14 being parametrized by the Eulerian rotation matrix RE(A, (Þ, E) R3(E) R2G — (b) R3(A), namely by three orientation parameters, conventionally called astronomical longitude A, astronomical latitude (Þ, orientation unknown in the horizontal plane E. {A, are the spherical coordinates of the local gravity vector r. See Figure 2 for an illustration. In terms of Cartesian coordinates being measured along the base vectors IF•, , respectively the local coordinates are transformed into global coordinates according to = RE(A., if we neglect any scale variation. Otherwise we have to extend 2(3) to a similarity transformation according to E. Grafarend, EH. Knickmeyer and B. Schaffrin (1982) including numerical examples. How can the parameters of the IF•—frame with respect to the IF•—frame be determined? Firstly, at least at the origin of the engineering network, we can determine {A, E} from astronomical observations at the datum point, from azimuth observations. Secondly we could determine {A, (i, E} from a threedimensional resection to three neighboring points which are coordinated globally, e.g. by GPS. For details we refer to E. Grafarend, P. Lohse and B. Schaffrin (1989). E.)
- 16. 15 Figure5: Local reference frame IV (Pa) =IF,•, IF} (Pß) —IE*, (PO global reference frame IF• ("Greenwich", "orthogonal to Greenwich in the equatorial plane of some reference epoch" "along the Earth rotation axis at some reference epoch"). "What is a geocentric datum and why do we need to use one?" Because the Earth is so misshapen, when creating a datum to suit their country/region, geodesists have traditionally ’positioned’ the Ellipsoid/Spheroid so that it best matched the Geoid over their country. These are commonly called Local or Regional Datum. (AGD was a regional datum for Australia.) The result of the use of Local Datums was that lines of longitude and latitude between different countries/regions would not connect evenly. These mismatches were commonly over 100 metres — no big deal in the non–digital era, but with the arrival of the very popular Global Positioning System (GPS) technology this disagreement was no longer acceptable. World-wide datums which would be used in all countries ⁄ regions began to be developed. These are commonly called Geocentric Datums.
- 17. 16 Local or Regional Datums This diagram represents the Australian Geodetic Datum which was created in 1984 (AGD84 or AGD). Note how the Ellipsoid ⁄ Spheroid has been ’placed’ over Australia to best identify longitude and latitude. If you used this datum in the northern hemisphere longitude and latitude would be very poorly identified on the surface of the Earth. Geocentric Datums This diagram represents the Geocentric Datum of Australia which was created in 1994 (GDA94). This datum was part of a world-wide datum which was compatible with the USA Global Position System (GPS). Figure 6 Figure 7
- 18. 17 Conclusion At the end as I mentioned above the importance of digital mapping and of the importance of the local and global datum which is the basic of it we mentioned that their advantages and disadvantages it seems to us that local datum sometimes better to be used but sometime global datum can we used. So for some places of the Earth it is better to use local datum such as the places that have a high relief and variation in typography because these places local datum represents its topography of this place better than global datum. So it depends on the places and countries of the globe .