Scale Factor and Convergence

Three optional output fields for Interactive and Point Database Conversions are Scale Factor, Convergence, and Orthometric Height Scale.  Scale Factor and Convergence only apply when a projected coordinate system is chosen as the output system.  Orthometric height scale will only display when a Vertical Reference is selected.  

[Grid] Scale Factor

Grid Scale Factor, often simply called "Scale Factor" is a measure of distortion at a given point on a projected map.  The scale factor is not cartographic scale, but a factor used to calculate actual ellipsoidal distances rather than distances on the projected surface.

Convergence

Convergence is the angle of difference in direction between Grid North and True North.  This will vary across a projected coordinate system and can be used as a measure of accuracy of angular measurements at a given point on the map.

Orthometric Scale

Also known as Elevation factor, Orthometric Height scale represents a factor of elevation that can be used to calculate geodetic distances above or below the ellipsoid (also known as reducing to the ellipsoid).  This scale is determined using a constant radius for the earth in the area of the calculation, but this is typically considered accurate enough for most applications.  

Combined Scale

Combined Scale is simply Grid scale multiplied by the Orthometric Scale.  This factor is used to calculate ellipsoid distance from a grid distance above or below the ellipsoid.

Example: The Grid Scale for two points is 0.99968900, and the Orthometric Height scale for the points is 0.999999123, the combined factor is: 0.99968900 x 0.999999123= 0.999688123

If the measured horizontal distance between the two points is 1000 meters, then calculate grid distance as: 0.999688123272747 x 1000meters = 999.69 meters