Purpose : Understanding Projections, 12D, and the System 1200.

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1 Purpose : Understanding Projections, 12D, and the System For any Cad work created inside 12D, the distances entered are plane (Horizontal Chord) distances. 2. Setting a projection, or changing a projection, in 12D from the Project menu does NOT adjust the coordinates. The projection set, enables the projection to be used in various 12D options. 3. By setting a projection and using the Geodetics menu inside 12D it is possible to enter Ellipsoidal distances 4. Reducing a TPS field file into 12D. If a projection is set using the Geodetics tab in the survey data reduction function the distances displayed the field file will be reduced in terms of the projection. Hence the coordinates calculated will be on the projection. By enquiring of a string (F2), the length will NOT be its horizontal field file length (It will be the join between the coordinates), If you enquire through Survey / Geodetics / projection bearing.measure the distance will be the horizontal measured distance. If the string is traced into the Traverse spread sheet the distance bought through will the ellipsoid distance (measured as per field file). If a coordinate system is used on the TPS1200, the coordinates calculated will be projection coordinates but the measurements held will be as observed (see also 6 below). 5. Reducing a GPS field file in to 12D. The coordinates are not effected by settings on the Geodetics tab. The Coordinates will be projection coordinates in terms of the settings on the GPS. 6. Setting the Scale Factor (SF) on the System1200. By setting a SF on a System 1200 TPS please note the following. a. Distances graphically displayed will have the SF applied. b. Coordinates calc d from the measurements will be reduced using SF. c. The original raw data of the observation will be recorded to the database. d. For example, i. Instrument set up at point A with Coords N=0, E=0 ii. SF of 0.5 set iii. Mark B, to the North, 500m away is measured iv. Graphic display of distance = 250m v. Recorded Distance to database is 500m vi. Coordinate in database, N=250, E=0 )(*+

2 7. TPS Surveying with SF. a. Manually booked Survey. It would be advisable to have NO Coordinate System attached to the job so that actual measured distances are graphically displayed. If the survey was at a constant elevation the GeomPPM could be set (as per illustration to right) to take into account the sea level correction, this correction would be reflected on displayed distances. Note Atmospheric and Refraction corrections could also be calculated and applied. b. Electronically booked Survey. The raw measured data is always recorded to the database. If the download is coordinates only, then the reduced coordinates as per 6 above will apply. If Observations are downloaded then no corrections will be applied, it would be necessary to apply corrections as per 4 above. NB: A 12D Field File Operation Code exists that can apply a Scale Factor to all measurements as exported out of the System This Op Code can be set to the GeometricPPM. Notes. 1. Cadastral Survey Guidelines Hardcopy Plans SURVEY PROCEDURES Chapter 3 - Page Cadastral Survey Dataset Distances The distances shown in cadastral survey datasets are to be ellipsoidal distances (Rule 22). In a CSD, this distance (also known as the spheroidal distance) can be approximated by the horizontal distance at mean sea level. These distances therefore reflect the actual ground distance after any sea level correction has been applied, are not projection distances, and therefore do not include the projection scale factor correction used to derive coordinates. Conversely, where a calculated distance is derived from projection coordinates and included in a CSD, the projection scale factor must be applied to correct the projection distance to an ellipsoidal or sea level distance. )(,+

3 2. 12d Manual Ellipsoid Distance The ellipsoid distance is the reduced distance along the surface of the ellipsoid. Standard survey measurements are reduced to the horizontal but require a correction due to the height above the ellipsoid. This is usually done by a height scale factor which takes into account the ellipsoid height at each end of the measured line. i.e. Where: h = Mean terrain height (mean of the two ellipsoid heights at either end of the measured line) R = Radius of the earth in the azimuth of the line. Note: An error of 60 meters in the value of h will introduce an error of 10 ppm in the reduced ellipsoid distance. With the introduction of the Australian Geocentric datums, the N values have increased markedly over Australia. So even if a survey is undertaken at sea level (Geoid height approximately 0.0), the ellipsoid heights may very well be greater than 60 meters. As The Ellipsoid height = Geoid height + N value, N values should be considered when reducing measured distances to these datums. For older Australian ellipsoids, Mean seal level approximated the surface of the ellipsoid (i.e. N value approximately 0.0), so corrections could use geoidal heights to bring the distances down onto the reference surface. The ellipsoid distance can be calculated using the calculated height factor: Ellipsoid Distance = Reduced Horizontal Distance x height scale factor. )(-+

4 3. 12D Manual Total station reduction Projection projection box current projection available projections the projection of the data to be reduced. If a valid projection is specified, the reduction will be done taking into account the projection scale factors. If this method is used it is paramount that the known coordinates (e.g station setups) are in terms of the projection coordinates and are not truncated (i.e. full coordinate values). These coordinates will allow the calculation of the relative longitude and latitude values which are used to compute coordinates from observations from the setup points. For more information about how to setup different projections see the section "Projections". N values choice box currently set method Available n value methods the N value method allows the conversion of non-ellipsoid heights to ellipsoid. The methods are defined in the project n value settings. For more information on the n value settings see the section "N values" The N value will be used to convert a geoid height (e.g. AHD) into an ellipsoid height. Ellipsoid height = geoid height + N value. The conversion is used for the calculations only. The original z value for the point will remain unchanged 4. System 1200 Newsletter 8 When the job is stored, a copy of the coordinate system is created and copied to the job. Again this means that this job can now be passed to another instrument and the required coordinate system is still attached to the job and will be used there is no need to have the same coordinate system stored on the second job. This is also an advantage when importing data from a job into an LGO project the coordinate system which was being used with that job can then be automatically attached to that LGO project the import is therefore very simple. )(.+

5 5. System 1200 Newsletter 9 STORAGE OF POINTS WITHIN SYSTEM 1200 The first and most important thing to remember with regards to the storing of points measured using the individual TPS1200 and GPS1200 instruments is the following: Points measured with a TPS1200 instrument are always stored with Local Grid coordinates in the DBX database. Points measured with a GPS1200 instrument are always stored with WGS84 Geodetic coordinates in the DBX database. Note, the two terms local grid and WGS84 geodetic should be considered as coordinate types (this newsletter will later introduce 3 other coordinate types which are needed to convert between local grid and WGS84 geodetic). It is only possible to convert between coordinate types if a coordinate system is being used. As described later, a coordinate system (typically) consists of a transformation, a local ellipsoid and a projection and should be thought of as the mathematical algorithms which allow the conversion between the two coordinate types. Note, even during the conversion of the coordinates to other coordinate types, the original measured TPS points always remain stored as local grid within the DBX and the measured GPS points always remain stored as WGS84 geodetic within the DBX. The mathematical algorithms allow the other coordinate types to be computed and viewed, but the original stored coordinate remain the same. )(++