6th February 2014, 12:36 PM
Interesting points! Scan distortion is something to watch out for, but like anything technical it can be alleviated with appropriate effort. Trouble is, if you buy in a bulk-scanning service it can be hard to implement effective Q-control. And if you use affordable flatbed scanners you do need to carefully choose to reduce the error (5mm on an A3 sounds a bit extreme...). Coming from an "urban" background where the average A3 sheet rarely holds more than a 5x5m patch, with grid points in all four corners (a hanging offense to omit these on MY projects!) the odd mm of distortion on the scan only blends in with the thick pencil lines and the slight uncertainties normally encountered when drawing features in a trench. Thus any error is trapped within each 5x5m square, and doesn't grow across the site. Planning at smaller scales (1:100?) to fit whole fields on a sheet magnifies the error, but then it also magnifies all the drawing errors too. And I never got on with those back-breaking A1 boards...
As for scans in CAD, AutoCAD only incorporates a rectangular "frame" and associated image path within the CAD drawing file - the actual raster image lives outside in its own file. The thing that really bloats the CAD file is all the digitised linework, since each line segment is defined separately in the file and needs to be "drawn" whenever the file is opened or zoomed or panned. But once again we return to Q-control and a firm sense of purpose. If the CAD drawing is the "master" dataset for a project it will have lots of extraneous stuff that will slow it down, but the "deliverable" products ought to be much smaller & more manageable exported sub-sets of the data, only enough to do the job for each intended drawing product. If your CAD system is somehow sucking in the whole raster image into the CAD file then you need to look at procedures & settings. For digitising (and rapid opening-times for CAD files) I also convert bloaty TIFFs to JPEGs - these are "working" copies so don't need to be archivally stable, and the visual effect of JPEG degradation is minimal.
And yes, I prefer doing "drawing" tasks like digitising in CAD rather than GIS, simply because CAD was developed as a drafting tool while GIS is more limited on the drawing-tools front.
As for scans in CAD, AutoCAD only incorporates a rectangular "frame" and associated image path within the CAD drawing file - the actual raster image lives outside in its own file. The thing that really bloats the CAD file is all the digitised linework, since each line segment is defined separately in the file and needs to be "drawn" whenever the file is opened or zoomed or panned. But once again we return to Q-control and a firm sense of purpose. If the CAD drawing is the "master" dataset for a project it will have lots of extraneous stuff that will slow it down, but the "deliverable" products ought to be much smaller & more manageable exported sub-sets of the data, only enough to do the job for each intended drawing product. If your CAD system is somehow sucking in the whole raster image into the CAD file then you need to look at procedures & settings. For digitising (and rapid opening-times for CAD files) I also convert bloaty TIFFs to JPEGs - these are "working" copies so don't need to be archivally stable, and the visual effect of JPEG degradation is minimal.
And yes, I prefer doing "drawing" tasks like digitising in CAD rather than GIS, simply because CAD was developed as a drafting tool while GIS is more limited on the drawing-tools front.