6th February 2014, 11:34 AM
I've used both methods in CAD and GIS over the years and personally, although I have to to use the scan method at the moment, I prefer digitising into CAD using a tablet. I've just found that CAD gives me more flexibility than GIS (Especially when comparing AutoCAD Map 3D to Mapinfo) and using tablets is quicker and rules out file management issues when trying to deal with the storage of hundreds/thousands of plan/section scans. But, at the end of the day, it simply comes down to what you are comfortable with and the combination of software/hardware at your disposal.
The only word of caution I have is to be aware of scan distortion. All our scanners, A4/A3, together with the commercial large format scan services we use, create a distorted image - usually more pronounced in the horizontal than the vertical, which although is negligible for individual sections/ trench plans, can have a significant effect on the accuracy for large area excavation plans if you are scanning in and digitising a large number of adjoining plans. The distortion tends to result in the grid on an A3 plan sheet being out by up to 5mm once it's scanned.
As you can image - if you have a hundred scans for an open area excavation and each scan is distorted by 5mm - you might begin to wonder why the digitised plans don't seem to match the GPS data .... I correct for this by setting up an accurate grid in CAD and then rubbersheeting the scans to the grid (I use AutoCAD Map 3D specifically for this reason).
I first came across the problem when a project manager wanted me to figure out why they were not only having problems tying in survey data to a phase of quarry excavation plans - but also why they were having problems tying in that phase to previous phases .... which also seemed to be 'a bit iffy'!
The problem with scan distortion is also the reason why it is never, EVER a good idea to either scan an object directly, or scan your pencil drawing for inking up digitally or printing out at a larger scale for hand-inking. I know of at least one large corpus of Anglo-Saxon material for a cemetery site which had to be redrawn at a late stage when the specialist realised the final drawings didn't match the dimensions he'd recorded - simply because the illustrator, to save time, scanned the 1:1 pencil drafts into the computer, re-sized them to the desired scale, and then traced over the resultant print-outs.
How about mentioning gvSIG?
The only word of caution I have is to be aware of scan distortion. All our scanners, A4/A3, together with the commercial large format scan services we use, create a distorted image - usually more pronounced in the horizontal than the vertical, which although is negligible for individual sections/ trench plans, can have a significant effect on the accuracy for large area excavation plans if you are scanning in and digitising a large number of adjoining plans. The distortion tends to result in the grid on an A3 plan sheet being out by up to 5mm once it's scanned.
As you can image - if you have a hundred scans for an open area excavation and each scan is distorted by 5mm - you might begin to wonder why the digitised plans don't seem to match the GPS data .... I correct for this by setting up an accurate grid in CAD and then rubbersheeting the scans to the grid (I use AutoCAD Map 3D specifically for this reason).
I first came across the problem when a project manager wanted me to figure out why they were not only having problems tying in survey data to a phase of quarry excavation plans - but also why they were having problems tying in that phase to previous phases .... which also seemed to be 'a bit iffy'!
The problem with scan distortion is also the reason why it is never, EVER a good idea to either scan an object directly, or scan your pencil drawing for inking up digitally or printing out at a larger scale for hand-inking. I know of at least one large corpus of Anglo-Saxon material for a cemetery site which had to be redrawn at a late stage when the specialist realised the final drawings didn't match the dimensions he'd recorded - simply because the illustrator, to save time, scanned the 1:1 pencil drafts into the computer, re-sized them to the desired scale, and then traced over the resultant print-outs.
How about mentioning gvSIG?
ShadowJack