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Helmerts Transformation - Disable Rotation

When fitting to an old astronomic plan I generally use a Helmert's Transformation. This causes problems when the rotation of the "local" points to the plan points is applied since bearings no longer match the listed plan. If you could disable the rotation and have a best fit translation it would help considerably.

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John Lunn shared this idea  ·   ·  Flag idea as inappropriate…  ·  Admin →


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  • John Lunn commented  ·   ·  Flag as inappropriate

    Hi Mark,

    I was hoping to make a best fit translation because if you compare the coordinates of the plan points to the local points it will not be the same shift for each set of points.

    The reason for this is that we are required to match astronomic bearings on older legal plans in the Land Title Office. In many instances these astronomic bearings can differ from survey to survey depending on where the bearings are derived from, survey errors, etc.

    Currently I shift the control onto the legal points which creates a coordinate system for that survey, if I use the same control and go to a different area once I shift the control onto the legal again it creates a new system that is just a little bit different than the first system. Each subsequent survey would require the same process causing the same point to have different coordinates depending on the legal information used to shift the control.

    The simple solution would be to put everything in UTM bearings. On plans that we can submit to the Land Title Office (subdivision plans, posting plans, etc.) we do submit using UTM bearings. It's more on topo plans, mortgage certificates, sketch plans, and other unofficial plans that we produce for clients that we must match astronomic bearings even though the control is in UTM.

    I was told that the field genius software had this option but I've never used FieldGenius myself so I am not to sure. I've also never heard of the clusters tool.

    Also I would have to go through the iterations myself if I found a few points that were disagreeing with the overall fit and decided to throw them out and calculate a new shift. This wouldn't be fun, and would most likely lead to errors. Currently I just rotate the control onto the legal points, I am looking to get away from that if at all possible so I can have everything on one coordinate system but still match legal plan bearings.



  • Mark Cahill commented  ·   ·  Flag as inappropriate

    The MSCAD Helmert's routine has alway had this option.
    Read the dialog carefully that pops up when you press OK to process.

    "Transformation Scale is:_________ Do you wish to scale the local points?"

    When you answer NO, scale is held at 1.
    If you answer YES, the scale factor computed, and displayed in the above noted message, is applied to your local point set as they are transformed.
    I hope this clears it up for you.

    Helmert's is a similarity transformation, and holds the "shape" of the trasformation set....which is to say the angles between points in the transformation (local) set are always held. Distances will only change if the scale points option, described above is applied...answering YES to the question.

    I'm not following the logic for your request? Do you wish to consider your local points to already be on the correct astronomic orientation?

    If this is the case, why not simply shift your points to the other system, then use the Clusters tool to generate a report on the fitment?

  • Ron Mak commented  ·   ·  Flag as inappropriate

    Along same line as Ben, allow Helmert to rotate but have an option to lock scale to 1.

  • Ron Mak commented  ·   ·  Flag as inappropriate

    Along same line as Ben, allow Helmert to rotate but have an option to lock scale to 1.

  • Ben Doherty commented  ·   ·  Flag as inappropriate

    Along the same lines, when trying to fit a plan to measurements, it would be nice to fix the angles, or fix the distances,

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