How measurements are handled
Cad systems can be considered as using their own units system which for the sake of differentiating can be called CadUnits, abbreviated to CU's.
Conventional measurement systems have unit designators (in, ft, mm, m … … ) associated with the numbers used to express their size. CU's are in fact unitless, identically sized drawing attributes but their physical size is unimportant. It is their number that matters. They are just attributes of the drawing that are used to represent numbers and, like a chameleon, appear to adapt to the environment in which they occur – in this case the measurement system that a user chooses to use. The CU's do not necessarily conform to any one of the conventional systems of measurement. When a user wishes to work with one of the conventional measurement systems they must inform the program of their choice so that the appropriate algorithms of the program are used when handling data entry and reporting in that system.
Cad systems generally use one CU for each real-world base unit of the measurement system being used and this works seamlessly while work continues using the chosen base units in that system. For example, when using any one particular unit from a measurement system as the base unit for a drawing, it is only necessary to enter the number of measurement units (without a units designator) so that twelve inches, feet, millimetres, metres, miles, kilometres … … and so on will in each case be represented in the drawing by twelve CU's. This means that the physical size of the drawing will be the same in each case even though greater or lesser dimensions are being represented.
When differing systems are involved
Significant difficulties arise, however, when a user needs to work in one measurement system with a drawing that has been created using another system. Now the user has to deal with the differences between the real-world systems of measurement. 12 inches is equivalent to 1 foot is equivalent to 304.8 millimetres is equivalent to 0.3048 metres … … and so on. It should therefore be evident that when a change of measurement system occurs different numbers of CU's will be needed to properly represent the model in the new measurement system. It should be further evident that the number of CU's needed using the new system will be in direct proportion to the relationship of the new real-world units to the original real-world units – that is a multiplier (factor) will need to be used to achieve the corrected new number of CU's in the drawing. That multiplier can be found by dividing the equivalent number of required new real-world base units by the current number of the old.
Examples (using the above figures):
Feet instead of Inches 1/12 = 0.08333... ; Millimetres instead of Inches 304.8/12 = 25.4 ; Metres instead of Millimetres 0.3048/304.8 = 0.001 ; Metres instead of Inches 0.3048/12 = 0.0254 … … and so on.
Scaling the drawing
Once the multiplier is known the whole drawing can be selected in DraftSight and the ''Scale'' tool applied using that multiplier to alter the scale of the drawing. This will alter the number of CU's in each entity of the drawing so that they agree with the number of real-world units that are needed to properly represent the model using the new unit system.
Where two drawings based on different systems of measurement are to be combined in a single drawing the scale matching can be effected by opening first in DraftSight the one based on the preferred measurement system. The second can then be embedded in the first before it is rescaled with the ''Reference'' option of the scale command so that two key points within it are scaled to map to two corresponding points in the first.
