The area covered includes 125 7.5-minute quadrangles lying at least partially within Baltimore City or one of the 16 coastal counties of Maryland. Shorelines located in out-of-state areas, such as the Virginia side of the Potomac River, were excluded.
In digitizing historical shorelines from hard copy maps, MGS had adopted an operational definition of the headward extent of a stream - the point at which the stream was represented on the map by a single line rather than a double line delineating opposite stream banks. That point, dependent as it is on the scale of the map, does not necessarily represent the head of tide. A similar criterion was used in extracting the 1990 shoreline. To the extent that the contractor could fit a double line on photography displayed at a scale of 1:24,000, the shoreline was extracted. Reaches upstream of that point were excluded. Again, the headward extent of a digitized stream does not necessarily correspond with head of tide.
Shorelines in the 1990 data set bounded several different wetlands classifications (Cowardin et al., 1979):
1. E 1 UB L (Estuarine -- Subtidal -- Unconsolidated bottom -- Subtidal)
2. E 2 US M (Estuarine -- Intertidal -- Unconsolidated shore -- Irregularly exposed)
3. E 2 US N (Estuarine -- Intertidal -- Unconsolidated shore -- Regularly flooded)
4. E 2 US P (Estuarine -- Intertidal -- Unconsolidated shore -- Irregularly flooded)
5. M 2 US M (Marine -- Intertidal -- Unconsolidated shore -- Irregularly exposed)
6. M 2 US N (Marine -- Intertidal -- Unconsolidated shore -- Regularly flooded)
7. R 1 UB V (Riverine -- Tidal -- Unconsolidated bottom -- Permanent tidal (tidally influenced, freshwater system)
8. R 2 UB 8 -- (Riverine -- Lower perennial -- Unconsolidated bottom -- Eusaline)
In addition to designating UB L and UB V as water, M 2 or E 2 US (Unconsolidated shore) M (Irregularly flooded) or N (Regularly flooded) classifications were considered water, given that those areas were more likely to be under water than above. E 2 US P (Irregularly flooded) was considered upland, given that those areas were more likely to be above water than under. In assigning rate-of-change attributes, MGS and CGIS devised and applied a number of rules, which are summarized below:
Classify shoreline segments of the shoreline type "structure" as "protected."
When adding nodes to distinguish between shoreline reaches with different rates of change, insert nodes only after the occurrence of a series of four or more similarly colored transects. For example, along an extensive reach that has experienced moderate rates of erosion, interrupted by two adjacent transects characterized by high erosion rates, classify the entire stretch of shoreline as "moderate." If, instead of two high-rate transects, six high-rate transects occur in a row, insert nodes on either side of the six transects, and classify that shoreline reach as "high."
If shorelines are so complex that it is difficult to distinguish between the two shoreline years or to see that they are properly positioned, assign the erosion rate category "Unknown." This problem is particularly common in marshy areas and/or along minor tributaries of dubious positional accuracy. A common manifestation of the latter is that one bank of the tributary is erosional along its entire length, while the opposite bank is accretional.
For reaches lacking transects, estimate rates based on the relative positions of the two shorelines. This situation is common along points of land projecting into the water.
Classify stretches represented by a single shoreline as "No Data."
If the two shorelines interweave over an extended reach, with little lateral displacement, classify the stretch as "No Change."