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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 Base Unit field can be set to Inch or Foot. 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 Inch, Drawing Uses Foot

If Base Unit is set to Inch but your drawing is measured in feet, the data points will be too spread out, as shown below. In this example, the grid spacing is set to 10 feet but data points appear every 120′.

DM Uses Foot, Drawing Uses Inch

If Base Unit is set to Foot but your drawing is measured in inches, the data points will be too condensed, as shown below. In this example, the grid spacing is set to 10 feet but data points appear every 10 inches.

The calculation will take significantly longer when Base Unit is set to Foot. If you’re not sure this is the right setting, cancel the command, change Base Unit to Inch, and run the calculation again.

How to Find Your Base Unit

If you’re not sure whether your drawing uses inches or feet, 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. For instance, if the width of a door measures roughly 36 units, the base unit for the drawing is an inch. If it measures roughly 3 units, the base unit for the drawing is a foot.

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