Units vs real world

Question

I've got a DXF (rev 10) CAD file with some 2D drawings and I'm implementing a reader. Until now I've successfully loaded everything and rasterized with ImageMagick.

But the point is, I have manually set the zoom on the coordinates to a number that made sense for me. How do I know what was the original size of the components and what unit was used to draw? Is there any specific group I have to look at?

My header is like this:

  0
SECTION
  2
HEADER
  9
$ACADVER
  1
AC1006
  9
$EXTMIN
 10
-14.610075
 20
-14.723197
  9
$EXTMAX
 10
14.556421
 20
15.530217
  9
$LTSCALE
 40
0.000394
  9
$PDMODE
 70
    35
  9
$PDSIZE
 40
0.000315
  0
ENDSEC

I've read what each part is about and I don't seem to find anything that helps me.

I want to know the units, because I want to be able to change the drawing accurately as it will be plotted, e.g. move a point by 2 inches.

Answer 1

When implementing a viewer for a dxf file, you don't actually need to know anything about the units. Unless of course, you are going to implement a Measure function in your viewer, then it gets more complicated.

Your initial 'zoom' size in your viewer can be determined from the header information that you have shown: EXTMIN and EXTMAX are the 2 key pieces of info you need. In your example the minimum coordinate use3d in the dxf file is -14.610075,-14.723197 and the maximum coordinate used is 14.556421,15.530217. This gives you a total drawing size of 29.166496(width) x 30.253414.

For a simple viewer, you can just assume that the units in the DXF file be equal to the units in your viewer (pixels or points or whatever you are using).

Then the base drawing size in your viewer will be 29.166496x30.253414, and you can scale that up (zoom) to make it fill whatever display area you have available.

EDIT

DXF files are by no means 'unitless', so in the case where you absolutely need to know the units, you will need to read the $INSUNITS group code value, and to double-check it, you can also read the $MEASUREMENT group code value.

The R2000 dxf spec, or any of the other versions, contain all the info you need on what those values mean. If you go to the 'HEADER Section Group Codes' page, and search for 'units', you will be able to find the listing of all the unit types. For example:

$INSUNITS
 70
     4

indicates that the dxf file is using metric units, specifically millimeters, as the base unit. So any dimensional or coordinate value stored by the dxf file will be in millimeters.

Default drawing units for AutoCAD DesignCenter blocks: 0 = Unitless; 1 = Inches; 2 = Feet; 3 = Miles; 4 = Millimeters; 5 = Centimeters; 6 = Meters; 7 = Kilometers; 8 = Microinches; 9 = Mils; 10 = Yards; 11 = Angstroms; 12 = Nanometers; 13 = Microns; 14 = Decimeters; 15 = Decameters; 16 = Hectometers; 17 = Gigameters; 18 = Astronomical units; 19 = Light years; 20 = Parsecs

EDIT

I just noticed you are using a very old dxf format (R10). If I remember right, the units were not introduced into the DXF spec until about R12. Before that time, the actual size of the drawing entities didn't change based on which units were being assumed. Only the labels on the dimensions were different from imperial to metric units.

If you are set on using the old R10 format, you will just have to make an arbitrary decision on what the units are; assuming you don't have any dimension labels on your drawings that would indicate what units are being implied.