Frequently Asked Questions

Why must I close the drawing currently open in AutoCAD before I can compare it to another drawing?

While calculating the Temporary Drawings, CompareDWG performs several operations on the input drawings. To make sure that none of these operations affect the original drawings, it loads the original drawings in separate drawing databases, which are discarded without saving when the comparison completes.
This requires that the original drawings are not opened in AutoCAD.

I compared two files, but no output is shown?

Either your input files were empty or no differences were found.

Does CompareDWG support DXF files?

Not directly, you should load them into AutoCAD and save them as DWG files, then use CompareDWG. Please be aware of precision issues.

If I change the definition of a block / linetype / layer etc., will this mark all entities that refer to this block definition / linetype definition / layer definition as changed?

No, if an entity keeps referring to the same block definition, linetype etc., then this will not be regarded as a change. Otherwise a small change to, for example, an important layer could mean that a large amount of entities would be seen as changed.
It is, therefore, important to check the Summary Report to find out whether blocks, linetypes etc. have been changed, then possibly check the Table Details Report to find out more.

Almost all of my entities are shown to be different, but I am sure most of them have not changed.

Most likely, one of the files has been stored in ASCII DXF format. This significantly reduces the precision of all real numbers. Specify a larger precision in the CompareDWG Preferences, then compare the files again.
Otherwise, you may have a drawing in which the handles have been changed. Try to compare in Basic Mode (more info Detailed Mode or Basic Mode).

I compared two completely unrelated drawings, but some changed entities are still reported?

The non-technical answer is: "Try comparing in Basic Mode, then this will not occur."
The technical background: CompareDWG identifies entities by looking at the space (i.e. Modelspace), the entity type (i.e. a Circle) and the handle (an internal ID number issued by AutoCAD). Since AutoCAD reuses the same handle numbers in each drawing, it is possible that both drawings contain "a line with handle 4567 in Modelspace", even if the drawings are unrelated.
This effect can occur if the drawings are completely unrelated or if the drawings are derived from the same parent drawing, and are then changed independently. The effect will not occur if one of the input drawings is derived from the other.
If the effect occurs, all differences will still be found, it is just that they may be reported as Changed where Deleted/New would have been more appropriate.

An entity is reported as changed, but I do not see any changes?

Use the Entity Definition command to see what has been compared. If you still don't see any difference, the colour may have changed, check the original drawings. Finally this can indicate a problem in the drawing database of one of the original drawings, try to use AutoCAD's Audit command on the original drawings.

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