Block Shift Transformation
For regional areas, the Block Shift transformation would be considered to be a very low accuracy method, but it may have application in small areas of interest where the accuracy of the shifts has been adequately defined relative to suitable control points in the area.
There is also an application for the Block Shift transformation in the conversion of local grid coordinates back to whole projection values again, where the leading digits of greatest significance had been removed for simplification. Another use may be to alter point heights by a fixed amount, for example after processing survey data where the height of a control point used for instrument setup was found to be in error, in which case the horizontal shift components would be set as zero so that only the heights would be changed. For this type of use, it might be necessary to set the type as Plane in Transformations and also to temporarily change the job coordinate type to Plane in Job Configuration.
|IMPORTANT - ALWAYS verify the accuracy and reliability of a transformation before applying to critical data - always test first with points whose coordinates are known in both systems.|
General Transformation Principles
General transformation principles will be found under the main topic for this section, Data Transformations - General.
Block Shift transformations are defined in the Transformations option under the Block Shift tab. The shifts may be defined as metres (Grid option) or as seconds (Geographic option) and may be applied to either grid projection or geographic coordinates.
For grid coordinates, shifts are simply added algebraically to the horizontal and vertical point coordinates - if the shifts are specified as seconds of arc, these are first converted to distance units before adding.
For geographic coordinates where the shifts are defined as seconds of arc, they are simply added algebraically. Where the shifts are specified in distance units and are less than about 50 metres, the shifts are converted to seconds of arc and added algebraically to the coordinates; if the shifts (metres) are larger, the point is first shifted in an east-west direction by rigorously computing a new longitude based on the distance shifted at an azimuth of 90 or 270 degrees, then shifted north-south by similarly computing the new latitude. This order of computation is necessary because a given distance in the prime vertical (along a parallel of latitude) subtends a different arc of longitude at different latitudes, so the longitude must be computed first.
The direction of the transformation is as defined by the Definitions library (in Transformations) but may be applied in the reverse direction if required - the signs of the transformation parameters are automatically reversed. Usually, the direction will be determined according to the selected transformation and the datum defined for the current job.
Points do not require valid heights unless a non-zero vertical shift is also applied. If the vertical shift incorporates a geoid-spheroid (N) value, no further value should be specified in the Default Geoid-Spheroid sep'n field so that no further adjustment to the point heights will be made if a different vertical datum is selected.
In selecting a suitable transformation, the source and target datums must match the datums for which the transformation is defined.
Further details of the transformation options will be found in the Transform Job topic.
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