Haphazard Hatches

[FitzUS]

I received a drawing from an architect that was used to to area summaries of an existing building.  They created hatches of each area and then summarized all the hatches by layer.  There was one large complicated hatch that at first appearance looked fine but when it was edited the resultant area dropped by almost half.  Doing a little investigation I made a copy, generated boundaries, deleted the original, and created a new hatch with the polyline.

The original hatch is shown above in Column A at the bottom, It was created using the "outer" option for island detection and reports an area of 20,655 SM.  The "re-creation" shown in Column B was made with the boundaries and using the "outer" option reports an area of 10,380 SM.  I couldn't figure out why the huge variation.  So I tried changing the island detection on each to see the result. 

The results in column A look way off no matter what island detection option is used.  I cannot figure out why these are so off.  Does anyone have an idea on what caused this hatch to give such erroneous area results.  I'm just curious so as to avoid this in the future.

The other question is the area difference between the "outer" and the "normal" options in column B.  These hatches were re-created and are closer in agreement but still 146 SM different which is about 1.33% off which can make a difference in large retail outlets.  Is there any way to establish which is the "correct" area?

Drawing attached.

[Yorx57]

After some fooling around (I just looked at the ignore) I was able to produce BOTH numbers (your column A and column B) using the exact same boundary.  The only difference was on how I actually selected the boundary - if I used the "select objects" option I get column A's number.  If I erase that hatch and re-do it again using the "pick internal point" option I will then get column B's number.  So it it appears it depends on how the boundaries where selected as to what it will report as the area.

[FitzUS]

Thanks Yorx-

First off, I admit the boundaries are poorly drawn.  The architect is a production house based in Hong Kong and they are notorious for poor quality control. 

I did a little more experimentation myself, mainly between the Outer and Normal options drawn in column B.  Those hatches I recreated from the boundaries generated by the original hatch.  I made two hatches, one with all the hatches that form the outermost boundaries and another that forms all the voids.  I then took those two values and subtracted (in Excel) the voids from the overall outer boundary and got the result that matches the "Normal" generation. 

I think it just shows that it pays off to draw careful boundaries, making sure there are no overlaps or gaps, select the boundaries to create the hatch and use the "Normal" island detection if you want to use hatches to do area measurements. 

I think that the original drafts person had difficulty making the hatch look correct and found an acceptable result by changing to the outer option.  This ended up causing an error in the area calculation of almost double the area, nearly 100,000sf. 

[Yorx57]

That's interesting that you were able to match the green "normal" value.  I ran another quick test this morning just to see what I would get. First, I did fix up the drawing at the top right - to the left of the "circle".  The 2 shapes there are actually made as 1 single one and the "divider" as another pline placed in the middle.  I then did a "normal" hatch and showed 10 524 320 098 as the area.  I then erased the hatch and used the "area" command with the add and subtract options (technically the same thing you did) and it reported 10 526 201 473.  So I'm getting a significant difference.  At this point, as we should be able to safely assume as shown in your drawing, there are issues with the hatch command and its reporting of the area.

Since we know there are differnet reportings and don't even know if any of them are correct, then even your way *may* be off as what it's reporting could be wrong still for all of the individual hatches.

So I would have to tend to believe with what the area command has reported as the proper area.