Subtracting area in field?

[Anonymous]

I know using the AREA command, you can subtract internal islands from an area. I place my area tags using field expressions in a block, referencing a PLINE. Is it possible to subtract internal islands from that area?

[cadffm · Accepted solution]

Field categorie FORMULA

[Anonymous]

Perfect, thank you!

[ChicagoLooper]

You can indeed use a field to display the area of a closed polyline. The area you want, however, the total area minus an island, or the net area, is not possible with that workflow, at least not efficiently. Instead of determining the area using a polyline, use hatch. If you hatch your closed polyline, then the hatch will have a net area, where total area - island area = net area. Put the hatch on a dedicated layer so you can easily freeze it. When  you stretch the polyline and increase the polyline's area, the hatch will increase with it and the field will update with the new expanded area. Simply perform a regen or regen all (RE or REA) for the field to update. You can also put your fields directly into mtext instead of using a block. 

[Anonymous]

I thought about hatches, but polylines are much faster to work with and doesnt clutter the drawing as much(while working in it). I think it's worth the trouble where I need to subtract, which isn't very often in these particular buildings.

Regarding MTEXT vs BLOCKS, they are attribute values that I export to csv files.