GIS and Aquaculture: Soft-Shell Clam Site Assessment
279
The storage of any spatial data within a GIS requires that location of the
objects be defined as either projection (metric X and Y) or degree (latitude and
longitude) coordinates, and the data are represented as points, lines or polygons. For this
study, ground control points (GCP) were used to generate a 100 X 100m grid on the two
estuaries (Indian Hole and Little Barasway) and the sand flats located in Big Barasway
(Fig. 2). The GCP(s) are marked in the field with surveying poles. These grids are
stored as polygons in an ArcView™ vector shape file. A unique numeric code, named
GEOCODE, is used to link the non-spatial information database (e.g. water depth,
bottom type, etc.) with the geo-referenced hectare polygons. In addition, the unique
code is also used for reference in the field. The map coordinates and the unique code of
each hectare grid centroid are stored as way-points in a global positioning system
(GPS). The GPS is used to location the grids for data collection and harvesting.
Therefore, because of the need to produce operational field maps, where the unique
numbers must have a logical base for referencing in the field, an arbitrary number
cannot be used for coding the grids. The numbers are based on the concept of a quadrant
whereby NE, NW, SW and SE quadrants are assigned numeric values of 1,2,3, and 4
respectively. Thus, if a 1 hectare grid is the first cell to the Northeast of the GCP it is
given the code 111 which means that the cell is located in the Northeast and is the first
row and column to the Northeast of the GCP. When the grid is located in the second
row to the north of the GCP and 20 columns away from the GCP in the Northeast
quadrant it would be code 1220. The first digit is the quadrant code while the following
digits represent the rows and columns.
279
The storage of any spatial data within a GIS requires that location of the
objects be defined as either projection (metric X and Y) or degree (latitude and
longitude) coordinates, and the data are represented as points, lines or polygons. For this
study, ground control points (GCP) were used to generate a 100 X 100m grid on the two
estuaries (Indian Hole and Little Barasway) and the sand flats located in Big Barasway
(Fig. 2). The GCP(s) are marked in the field with surveying poles. These grids are
stored as polygons in an ArcView™ vector shape file. A unique numeric code, named
GEOCODE, is used to link the non-spatial information database (e.g. water depth,
bottom type, etc.) with the geo-referenced hectare polygons. In addition, the unique
code is also used for reference in the field. The map coordinates and the unique code of
each hectare grid centroid are stored as way-points in a global positioning system
(GPS). The GPS is used to location the grids for data collection and harvesting.
Therefore, because of the need to produce operational field maps, where the unique
numbers must have a logical base for referencing in the field, an arbitrary number
cannot be used for coding the grids. The numbers are based on the concept of a quadrant
whereby NE, NW, SW and SE quadrants are assigned numeric values of 1,2,3, and 4
respectively. Thus, if a 1 hectare grid is the first cell to the Northeast of the GCP it is
given the code 111 which means that the cell is located in the Northeast and is the first
row and column to the Northeast of the GCP. When the grid is located in the second
row to the north of the GCP and 20 columns away from the GCP in the Northeast
quadrant it would be code 1220. The first digit is the quadrant code while the following
digits represent the rows and columns.