Page 299 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

278 Maritime Archaeology: A Technical Handbook, Second Edition
into 4 or 16 subunits. Thus when an object is found, the excavator can iden- tify the position by noting the coordinates of the object on the identified grid frame, or where the precise location is not relevant to the grid frame, the number and the sub-grid can be identified. It is worth considering before starting excavation what the coordinate system will be used, Figure 10.4 shows the problems likely to be encountered if one selects the mid point as the origin, where one will have positive and negative coordinates. As dis- cussed in Chapter 9, because the grid frames are likely to move around slightly, the decision to use the grid frame as a control for recording will need to be carefully thought out. It is likely that a more rigid system will be required for accurate recording and a strategy for different types of objects decided upon.
However, some artifacts are unsuitable for tagging because they have no convenient point at which the tag can be attached (e.g., a cannon ball). Such objects can be placed in a bag with the tag. If the bags are coded, then the need to tag can be averted simply by noting the bag number and its
Y
WRECK
SITE
0
X
X
WRECK
Y
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SITE
Figure 10.4 A schematic showing two different types of grid coordinates. In the upper version the origin is centered off the site so all X and Y coordinates are positive. In the lower example the origin is in the center of the site creating positive and negative X and Y coordinates.