More about scale factors

By Doug Aaberg • April 8th, 2019
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One of the more frequent questions I have received so far this year has to do with combined scale factors and grid-to-ground conversions. As surveyors increase their use of GPS-RTK measurements for preparing plats, clients are requesting more detailed information regarding those measurements. If a surveyor simply puts a note on their plat stating “all measurements are grid distances” the end user may very well want to know how to make sense out of that. A lay person will understand a measurement shown on a plan as something they can reproduce on the ground at any given time. A grid distance however, needs to be converted first. Therefore, the scale factor used would need to be denoted on the plan.

As more states move toward low distortion projections, the need to reduce grid-to-ground distances will diminish. Although I am not sure exactly how low distortion projections will take into account massive elevation changes across a site, the thought of not having to worry about converting grid-to-ground distances will likely be a welcome change for many surveyors. In the meantime, for those who need to deal with converting their field measurements to ground or their plan measurements to grid, Carlson has an easy solution.

    Setting up a combined scale factor

If you have performed a survey using a typical GPS unit, chances are you are defaulted to grid distances, whereas your total station is likely defaulted to using ground distances. Carlson’s SurvNET program is able to adjust a network using both total station measurements and GPS vector data. That is a subject for another blog, but if you have not used SurvNET to date, it is another great Carlson tool.

  1. To setup a drawing for a grid to ground conversion, begin with the Drawing Setup.
  2. Set the Coordinate System to Grid followed by the Projection and Zone.
Note: This command does NOT alter the coordinates in the crd file. This setup routine allows the user to identify and report coordinates and distances in a variety of ways but does not rescale existing coordinates. That process can be accomplished through the Coordinate File Utilities dialog box.

 

Note the Report Distance Scale options:

  • Fixed will report distances with a single scale factor supplied. The default is 1, meaning a distance measured between two points will display as a direct inverse measurement.
  • GRD>GND will scale a measured distance from grid to ground based on the supplied projection and zone.
  • GND>GRD will scale a measured distance from ground to grid based on the supplied projection and zone.

The GRD>GND and GND>GRD options are very unique from the perspective that they allow you to measure and annotate distances on the fly at any point on the site you have surveyed. The combined scale factor will be different for each location but can also be annotated along with the distance. Very simple!

The Fixed option allows you to apply a single combined scale factor for the entire site. This makes it easier for the end user to convert distances shown on the plat in either direction.

  1. To use this option, click the Set button to the right of Factor:
  2. In the Distance Scale Factor dialog box, click Calculate Combined Factor.
  3. Next, enter the range of points you would like to use to calculate the combined scale factor.
Note: Only use points with actual elevations. Do not use points with a 0 elevation or non-surface points as these will distort the final calculation.

 

  1. Set the Scale Direction to either Grid to Ground or Ground to Grid.
  2. Click Calculate followed by OK

The command calculates the combined scale factor and populates that value into the Factor dialog box

Note also the Rotation option. This setting allows you to annotate bearings along a line that is based on a different basis of bearings than the original survey.

 

    Using a combined scale factor

Once you have set the combined scale factor, you can make use of it several different ways.

Inversing point

  1. To simply inverse between any given 2 points, use the inverse command and type O for options.
  2. Toggle the Report Second Scaled Distance on, and add a suffix for the distance.

This should correspond to the settings in the drawing setup i.e. grid to ground or ground to grid.

The resulting display will be similar to below:

In this case, the survey is based on grid distances so the second scaled distance is a ground distance.

Annotating distances.

Similar to the inverse command, you can add the second scaled distance when annotating a plan.

  1. In the Annotation Defaults, set the 2nd Scaled Distance Option to Label 1st and 2nd
  2. Add an optional suffix to the annotation
  3. If desired, enable the Label Scale Factor option and set a prefix for this annotation.
  4. Set the Decimals for Scale to an appropriate precision for the combined scale factor.

When annotating a line, the results will look similar to below:

Listing Points

You can also list individual points with a lot of additional information beyond just P,N,E,Z,D.

    Select List Points

  1. Enable the Use Report Formatter option
  2. Enable the Include Geodetic Report option
  3. Click OK
  4. Select Geodetic Options and set all options as desired.

 

  1. Select the desired report fields from the left side (available) to the right side (used).
  2. Click Display

The report displays the information selected for each individual point. Note the calculated combined scale factor for each point.

Labeling Areas

Since the survey performed in this example is based on grid distances, you also have the ability to calculate and label geodetic areas as well.

  1. Select Area Defaults
  2. In the Area Defaults dialog box, select the geodetic options from the left available fields to the right used fields.
  3. Select a method to label areas such as Area by Closed Polylines
  4. Select a polyline in the drawing and following the prompts, label the area.

The resulting area shows both the default area as well as the geodetic values.

Some users may very well be surprised just how much difference they see between their grid and ground measurements. For that reason alone, it is worth taking the time to set up your drawing to calculate both dimensions for a survey. Then you can decide just how you would like to report those differences, if at all. Either way, Carlson will make the process easy for you.

Please consider coming to the 2019 Carlson User’s Conference being held on May 7th through 9th in Maysville, Kentucky. I will be teaching several classes. You are welcome to attend and further discussions like these.

http://www.carlsonsw.com/community/uc19/

Doug

Let’s Grow Together.

Douglas L. Aaberg, PLS

Survey Product Manager

P)617-393-2300×419

daaberg@carlsonsw.com

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