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Base Units and Photometrics Calculations

Tuesday, November 19th, 2019

Q: When I run the photometrics calculation for an area, the foot-candle data points are more condensed/spread out than they should be. What would cause that?

A: This usually happens when the base unit for the calculation is not set correctly. In the Calculate Photometrics dialog box, the Distance Unit field can be set to Inch, Foot, Millimeter, Centimeter, or Meter. This setting must match the base unit for your drawing.

Base Units Match

When both the drawing and the calculation use the same base unit, the results are as expected. In the example shown below, the grid spacing for the calculation area is set to 10 feet and the data points appear every 10 feet.

DM Uses Base Unit Smaller Than Drawing

If Distance Unit is set to a smaller unit, such as Inch, but your drawing in a larger unit, such as feet, the data points will be too spread out, as shown below. In this example, the grid spacing is set to 10 feet, but because DM  is treating each unit as being 12 times smaller, data points appear every 120 feet.

DM Uses Base Unit Larger Than Drawing

If Distance Unit is set to a larger unit, such as Foot, but your drawing is measured in a smaller unit, such as inches, the data points will be too condensed, as shown below. In this example, the grid spacing is set to 10 feet, but because DM is treating each unit as being 12 times larger, data points appear every 10 inches.

The calculation will take significantly longer when the Distance Unit is set larger than the drawing units. If you’re not sure whether Distance Unit is set correctly, cancel the calculation, change Distance Unit to a smaller unit, and run the calculation again.

How to Find Your Base Unit

If you’re not sure which base unit your drawing uses, you can find out using standard CAD commands:

  1. Run the UNITS command.
  2. Make a note of the current Length Type setting, then change it to Decimal and press the OK button.
  3. Run the DIST command and use it to measure a dimension on the drawing that you know, such as the width of a door or parking space.
  4. (Optional) If you changed the Length Type earlier, return it to its previous setting.

Use the distance measured to determine the base unit for your drawing. The following list uses the approximate width of a door as an example:

  • Inch: 36 units
  • Foot: 3 units
  • Millimeter: 900 units
  • Centimeter: 90 units
  • Meter: 0.9 units

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