The ability to draw directly on a tablet is one of the features which originally lead to paperless archaeology. Many excavations have tables and umbrellas set up for trench supervisors to update databases and write notes. But scaled drawings have always been a ‘grab the board and jump into the trench’ affair. You have to be right on top of the items being drawn. And mice or trackpads just do not work as good input devices for drawing in the field.
Our current workflow for field drawings center around our use of TouchDraw on the tablet and its ability to import/export those drawings to our CAD environment (using SVG as an intermediary format). This page describes that workflow.
TouchDraw has a couple of features that make it ideal for field drawings. It has an unlimited paper size and ridiculous zoom levels. These two features actually allow us to remove the ‘scale’ from scaled drawings and draw at 1:1 on the tablet. Since we can draw at 1:1, we can export the drawings from TouchDraw, and import them into AutoCAD where the features of the drawings can be spatially corrected against EDM survey points, and the SUs can be transferred to the CAD drawing.
We start with a template in TouchDraw. This has pre-defined layers with a scale bar, title box, and a box ready for typing elevations.
The layer structure is
- Title Elev
- Trench Outline
- Scale bar
That template is loaded onto the iPad. Also loaded is a copy of the field drawing library, which contains pre-formatted text boxes and elevation marks for easy loading onto the drawing.
If there is no base plan for the excavation, drawing can commence right away. Establish a baseline for measuring on the ground and in the baseline layer of the drawing. To keep things organized make sure that drawing elements are in the correct layers. This will enable you to lock layers or toggle them on or off.
For our purposes the walls layer is for ground level of standing architecture. Since we cannot remove these walls they help to serve as reference for measuring. The trench outline layer can be used for the same purpose. We separate out our SUs and features. While features do get SU numbers, separating items such as floors, in situ stones, cuts, and other such elements allow us to later isolate those elements in the drawing. The labels layer is for recording the SU numbers and elevation marks.
The excavators at PARP:PS start with base plans. We start with a portion of the basic plan of Pompeii. We only use a portion because we work in a local coordinate system and only work with our general neighborhood. The walls in that plan need a little bit of correction which is provided by the total station. Once the walls are placed correctly in the CAD plan for the room which will be excavated, we plan where the trench outlines and baselines will be, and shoot those points in with the total station as well.
Our CAD environment is AutoCAD for Windows, and what we need AutoCAD to do is export a SVG file that can be imported into TouchDraw. We use DWG to SVG Converter 2011 MX by DWG Tool Software to do this. In order to keep the SVG file size small, we copy the objects necessary for each of the base plans: wall bases, architecture, trench outlines, baselines, and four target points to a new DWG so as to remove the extraneous data. The drawing is rotated so that the main baseline in each trench is either straight vertical or horizontal (to help the field artists use the TouchDraw background grid for measuring). In the options menu in the DWG to SVG converter we select “Last Saved View” and adjust the output size to match the view dimensions in the CAD model. If we are viewing plan of trench 53000 for instance the dimensions of the view window could be 26.55×11.62m and the image size we would output would be 2655×1162. The resulting SVG has a scale of 1px:1cm and is small enough, <500kb, to be easily manipulated in TouchDraw.
output from CAD for trench 56000
The SVG file is imported into TouchDraw and transferred to a blank template mentioned above. The elements are separated out into their proper layers and assigned our standard line models (light blue for baselines, orange dotted for trench outlines, and black for walls/features). Those layers are then locked to prevent unintentional editing.
Plan with layers in TouchDraw.
When the base plans are ready for the excavators the only layers that are unlocked are SU, features, labels, and title elev.
The base plans are duplicated for the drawings, they are not drawn upon directly.
As is the case with paper scale drawing, digital drawing takes some level of skill and is usually taken over by one or two excavators who seem to have a knack for the task.
The basics are the same as paper. Measure from a known feature or baseline, duplicate that measurement on the device (with either a single line or rectangle), and use those measurements to draw a freehand line between those points. Once the line is drawn, you can move, add, or delete vertices to get the right shape. It is best to draw the SUs and features first and lock the SU layer before labeling. The elevation marks are dragged from the TouchDraw library and the numbers are changed to the proper elevation number.
Once one drawing is complete, it is duplicated for the next drawing. This keeps any of the SUs, features, and labels in place and ready for reference. Drawing elements that are no longer needed are deleted, and new ones added. It is helpful at this stage to know that not only can you lock entire layers, but you can also lock individual drawing elements in TouchDraw. So drawing one SU against another SU that already exists in the drawing need not be too complicated.
Items that are needed from an earlier drawing are easily copied and pasted into the new drawing. TouchDraw pastes spatially, much like the Paste in Place command in Adobe Illustrator, so items get pasted in their proper place. This makes phase drawings at the end of the season much easier to complete.
The drawings follow a certain naming scheme. For us that is trench drawing # (53000 plan 5, 53000 profile 2, 53000 phase plan 2). All of the drawings are saved from the tablets each day, but are only exported to CAD when the trench supervisors announce that the drawing is complete.
The drawings reflect the state of the trench on a certain day. Therefore features that stay in place during that day stay in place in the drawing. Features that are removed are removed from the drawing. New features are sketched in the plan, and then their location is recorded by the total station. Once the drawing is imported into CAD, the sketch is snapped to its exact location. We then export an entirely new base plan for the trench supervisor so that they can be confident in their use of the feature for measuring other elements in the trench.
Importing the Drawing to CAD
When the drawings are finished, they are saved in the native TouchDraw format. Then the Walls, Trench Outline, and Scalebar layers are turned off. The plan is exported into SVG format with only the registration points, baseline, and any features or SUs drawn in. The plan is then exported back into svg format and loaded into Inkscape, which is compatible with both SVG and DXF, then saved as a DXF. The DXF can then be opened by AutoCAD, the objects can be rotated and scaled quite quickly using the target points and then elements are copied into the DWG.
The native svg output and TouchDraw files can be downloaded from here.
This page was written with the help of Greg Tucker of the BSR, who is our new project CAD developer.