Leica quick-guide-determine-coordinate-system-normal-method

Determine Coordinate System – Normal Method (Multiple Points)

Determine Coordinate System – Normal Method (Multiple Points) Determine Coordinate System using the Normal Method is mainly used for sites that already have existing control points. A survey using GPS equipment of the existing control points needs to be carried out. Then the GPS LLH coords matched to the known Local ENZ coords. Using the matched coordinates a projection is created with the origin at the mathematical centre of the site transferring the points from the curved surface to a flat grid with coordinates (0,0). Then a 2D translation, rotation and scaling creates a best fit between the two sets of coordinates and gives the projection origin a set of false coordinates. It is possible to hold the scale at 1.000 so that the true geometry of the GPS observations is held rather than distorted to fit the given coords. The height conversion parameters are created by an average difference between ellipsoid heights and elevation above datum being calculated. In addition to this a plane is fitted through both sets of coordinates. The difference of inclination between the planes is calculated and is applied in addition to the constant difference as a ppm value along the easting axis and the northing axis. Below are the steps to follow to determine a coordinate system from existing site control.

Step Instruction Screenshots

1

Create a new job and key in the known coordinates for the control 5.Manage > 1.Jobs > F2.New > “Name” > F1.Store > F5.Data > F2.New > F1.Store Repeat last two steps until all points are keyed in

2

Use your normal method of surveying to capture these points. However as they are control points you may wish to observe each point for longer or take multiple observations and average the set. To increase occupation times select User > F2.Config > 1.Survey Settings > 6.Point Occ Setts > F2.Param To take multiple Observations select F2.Near or Shift > F5.Individual Pt ID after each observation to change the point ID to be the same as previous point

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3

Run the Determine Coordinate System (DTC) program. The DTC Begin screen that appears will request a coordinate system name, the jobs containing the observed WGS84 coords and the known ENZ coords and the method to be used. Set the method to be Normal

4

The Step 1 screen confirms the name of the coordinate system and then the type is requested. This is set to Onestep for small local sites. Onestep generated the projection in its simplest form from the two given set of coordinates. For height mode select Orthometric. This will report the levels relative to the local datum, rather than to the ellipsoid.

5

The Step 2 screen asks you select a Geoid model to control the levels. If the levels are to be reported relative to a local site datum such as a peg in the corner of site select “None.” However if the datum is a national datum such as the OS Newlyn datum select the relevant Geoid model. Eg OSGM02

6

The Step 3 screen is for matching the control points that coincide with each other. If the point names match exactly in the two jobs selected at the Begin screen, press F6.Auto. All points of matching names will be paired. If the names differ then the matching will need to be done manually by pressing F2.New.




7

WGS84 point requests a point from the observed list and Known point requests the equivalent point in ENZ. Note that when WGS84 point is highlighted the option to Survey (F5) the point is available. It is also possible to key in values for both sets of points here too. Simply press the Enter button to gain access to the Data Management Screen. Match type controls if the point will have an influence on the Plan, Height, both or neither of the DTC

8

When all the points are paired you will have the options to change the F5.Match status of each point. Once happy with this select F1.CALC to compute the parameters of the projection and the height transfer.

9

The Step 4 screen reports residual values in E,N and Z of how well the GPS observations with the CSYS parameters applied fit to the known coordinates. There will always be residual values but they should be no bigger than 25mm.The worst values will be highlighted with ! F5.More will scroll across to show the Z residuals. If any residuals are too high ESC back a page and alter the Match status of the point and then re-calculate

10

Pressing F3.Result will show the Results of the DTC. The information shown is the Shift XY to generate the false origin and the rotation of the points about which point. The Height tab shows the difference in inclination of the two planes along the X and Y axis together with the constant height difference applied to Ellipsoidal heights to gain local datum levels.



The Coordinate system can now be stored by Pressing F1.Cont to access a summary page and then F1.Store. The Coordinate system is then available to be applied to any job. Coordinate systems can be transferred from one set of equipment to another by utilising the Transfer Objects function. Select 6.Tools > 2.Transfer Objects > 03.Coordinate Systems. Set the From field to be System Ram and the To field to be CF Card and select the coordinate system that you wish to transfer. Move the CF card to the second set of kit and do the reverse by switching the From and To fields. NB: many of the fields selected in Steps 1 to 4 can be set as default by selecting F2.Conf whilst in the Determine Coordinate System Begin screen.


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Leica quick-guide-determine-coordinate-system-normal-method