Using Spatial Calibrations

Using a spatial calibration, Image-Pro's pixel-level measurements can be converted to any unit of measurement.  Spatial calibrations can also correct for irregularities in the image’s vertical and horizontal spacing.

There are two primary components to a spatial calibration:

n     Pixels per Unit

n     Aspect Ratio.

Pixels per Unit

A spatial calibration tells Image-Pro the size to which a given image is scaled, just as the key of a roadmap tells us how many millimeters of image length represent one mile.  Likewise, the spatial calibration tells Image-Pro how many pixels of image length represent some more meaningful unit of length in the image, for example, one micron, millimeter, or inch.  To create a new spatial calibration, you must either know the number of pixels per unit to specify for the image, or there must be a feature pictured in the image that has a known length value in terms of the units of interest.  If a feature of known length is pictured, Image-Pro can calculate the number of pixels per unit by determining the number of pixels it takes to represent the known length.  (Companies often ‘plant’ an object of known length into images for this purpose.) 

Aspect Ratio

Aspect ratio refers to the ratio of vertical to horizontal lengths.  For example, when a television screen’s image appears flattened or squeezed, the ratio of horizontal length to vertical length is out of proportion.  Similarly, cameras often inadvertently skew images in the process of capturing them and translating them to digital format. 

Such skewing results in inaccurate lengths being ascribed to features that are measured in the image.  When a skewed aspect ratio has a flattening effect, vertical lengths will be recorded as shorter than they actually are; when it has a squeezing effect, horizontal lengths will be recorded as shorter than they actually are. 

If an image has a known aspect ratio problem, Image-Pro can compensate for the flattening or squeezing of vertical and horizontal lengths, and calculate the actual number of units that measurements of its features represent.  To do this, you must be able to supply it with the value l/h (length divided by height) for a perfect square as represented in the image, or there must be a feature pictured in the image that represents a perfect square.  If a square feature is pictured, Image-Pro can calculate the aspect ratio from that feature.  (Companies often ‘plant’ a perfectly-square object into images for this purpose.)