Check the sketch: getting down to detail

I recall being called an ‘old school’ surveyor by a survey manager in a suit (who should have known better) when I suggested the site sketch is still the ‘last best resort’ in mapping building deatils. Since then I have often wondered if there is a better way of getting the small and awkward bits of building plans done in the digital age. So far I have not found it- the measured sketch is fast, accurate and CAD ready. The time it takes to extend control into cupboard size spaces is out of all proportion to the amount of information captured- same goes for the scanner too. I know a lot of surveyors avoid drawing like the plague but with a little practice, and a systematic approach, a good site note is worth its weight in gold.

There is a balance to this and usually I find site sketches need to be done when the job is almost complete: only then can you see the bits that need the extra attention. Certainly it’s a slow method compared to direct to CAD from Disto or TST. Some things just need to be drawn anyway- windows and door casings seem to need the clarity of a site note especially in historic buildings where ‘standard’ blocks won’t do.

Plotting a site note into CAD is a bit of a pain and I make sure I always have some means of tying in the hand measured work such as a trace of the outside of  a window or door or a TST  ‘crows foot’ in the corner of a tight space.

It occurred to me that part of the ‘ fear of drawing’ might be a gap in understanding how to plot from the sketch into CAD  so here is a quick FAQ based on what I have hacked together out of the mountain of options CAD throws at you : I expect there are better ways but these are the bits that work for me:

Plotting a site note into CAD FAQ

1. How do I place lines ‘square on’ to ‘non- square’ walls?

The obvious answer is to use the ‘snap to perp’ but I find it a bit awkward  as you need to find a start point before you can snap the end point ‘perp to’ an existing line. This may seem a bit convoluted but I work in a local UCS to my base line and the use (F8) ‘ortho’ function. The key to this is to make sure you have a level line as the base line: trace over any line that might have z variation from one end to the other with a (2D) polyline then use the polyline as the base line. Select the UCS command, select ‘e’ for entity or ‘o’ for object and pick the polyline. Check the orientation of the UCS axes: you want Z to be ‘up’

so type UCS, hit ‘x’ and accept the default rotation of 90…

Command: ucs
Current ucs name:  *NO NAME*
Specify origin of UCS or [Face/NAmed/OBject/Previous/View/World/X/Y/Z/ZAxis]
<World>: x
Specify rotation angle about X axis <90>:

…that’s better and you can now work at the newly aligned  orientation with F8!

2. Is there another way to get the distances for the lines in other than ‘Offset’?

Yes! Just hit F8 (ortho),  pick your start point, drag the cursor the way you want the line to go and key in the distance, drag the cursor again, key in the distance etc…It’s what I call a ‘hanging line’  technique; the end of the line hangs off the cursor and the line is fixed by the distance keyed in. ( This can be handy if you are plotting with a live Disto too!)

3. The TST measured lines are 3D. How can I get the drawing to work in 2D?

There are 2 routes for this:

The first is the ‘flatten’ command; this will smash the whole drawing flat so its a bit destructive if you want to use any height info for anotataion or working up sections later on. I make sure I follow the command with a ‘save as’ and rename the drawing to keep the measured data for future reference.

The second method is to use polyline (as at 1 above) to trace off the 2D lines in the plan UCS. A polyline will always have the same Z value at one end as the other so if you snap the start point onto a piece of 3d wire it’s next point will be at the sme Z.

4. How can I get dashed and dotted lines to work properly?

It’s all down to the Bricscad/AutoCAD variable LTSCALE (linetype scale) …… by default this will be set to a value of 1.000. It works in 2 ways: as a global setting  and as an idividual proprty of an object. I have never had much luck with the global setting approach so I work on the individual object properties and set it to betwen 0.003 and 0.007 for drawings created with a metric template where 1 drawing unit=1 metre.

5. Blow this for a game of soldiers can’t I just trace off a photo?

If you have good control points, a decent stand off and illuminate the wall evenly you can…but if things are tight and the lighting is against you snap away… but get the sketch too!

your sketch will let you fix measurements across the whole wall as well as remind you of where the photo goes: which all makes sense when you bring them together in CAD:

I can use the photo for the bits I can photograph and the sketch for the bits that won’t:..the wonky floor and window profiles were traced byTST with TheoLt, the window glazing detail traced from a photo rectified with with Photoplan and all else was by measured drawing.

6. Plotting diagonals

It’s a case of creating the required arcs and findng the instersections. Easier said than done. The trick here is to make sure the circles are all on the same Z- the intersections won’t work otherwise! Use the Break command if you have to (when double intersections occur) and then trim the arcs against each other.

7. Is sketching on a tablet PC a good idea?

Yes! but I have found it surprisingly hard work to keep things to scale as I trace with the pen into CAD. I think this is to do with the ‘scale free’ effect of zoom in CAD.  Although you will save office time sketching into CAD on site I have found it far quicker to use a pencil on paper and get things to scale back at the CAD lab!

8. My desk is covered in paper the last thing I need is more of the stuff!

Indeed, get a bigger desk…or an A0 drawing board perhaps? I jest. The answer is to scan (or photograph) your site notes and then you can attach them to the DWG as image files and take them with you on your tablet or laptop! Scanning the notes takes a bit of practice but I have found a 50% scale scan at 200dpi is usually good enough for my(A3 size) notes. I do them as greyscale too to save on file size and loading speed.

So there you are: quality building surveys still need sketches…and photos…TST traces… and scans and…

B

About billboyheritagesurvey

Heritage worker
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