Creating a New Shape in Saga from Published Coordinates
Abstract: In this
exercise you can use a file of existing geographic coordinates, such as
the boundaries of a marine area, to create a standard shapefile.
The results can be points, lines or polygons, according to your need. For
de novo digitizing of physical coastlines or edge features (i.e.
getting coordinates from from
images), you should
use the exercise
2.26 Digitizing High-Resolution Borders and
Coastlines in Google Earth.
Preliminary Reading (in
OceanTeacher, unless otherwise indicated):
Brown with Volker Wichmann
An excellent tutorial on how
to digitize a shape without any existing digital file is found at
|1. Open and read through the
Garden Banks website, covering some really wonderful reefs in the Gulf of
Mexico. It includes maps for the different banks that make up the
Sanctuary. We're interested in Stetson Bank, shown here.
coordinates, as is true of all national and international marine protected
areas, are established by law and must be quoted and used exactly.
|2. To convert the coordinates
to usable signed decimal degrees, make this Excel spreadsheet.
You can see
the easy formula to change each value to "original" format to "decimal" format:
- Plus minutes/60
- Plus seconds/3600
With the overall signs given by the hemisphere of the original data.
Negative longitudes in this case.
Note that the first column contains integers. Only one polygon is
included here (Stetson Bank), and it is arbitrarily named 1. Only the
polygon corners are needed (4 in this case), and the first coordinate is not
repeated at the end to close the polygon. Saga does that
|3. Save these
- stetson_bank_coordinates.xlsx - Excel spreadsheet; for
possible future editing
- stetson_bank_coordinates.txt - Tab-separated ASCII for import
4. In Saga, select FILE > TABLE > LOAD TABLE and load
the TXT file you just made.
|5. In Saga, select TOOLS >
SHAPES-TOOLS > GENERATE SHAPES.
|6. This description indicates
that you can make points, lines or polygons. The input must include X,
Y and IDENTIFIER columns.
|7. Here the proper choices
have been made.
- If you selected LINES you would get a multi-sided
shape that is not closed.
- If you selected POINTS then you would get a
set of separate, unconnected points.
- We chose POLYGONS to get a closed
|8. And here is the polygon
shape, color-filled, which is a characteristic of new polygons.
|9. And here is the polygon
mapped in Saga. You can see the Texas coast (from World Borders) along
the western side of the map. Great things come in small packages.
|10. Make sure to
save the new shape with an appropriate name in PRODUCTS > SAGA > VECTORS.
|11. The TXT file
can contains more than one polygon.
Each separate polygon is
identified by unique integer identifiers; below they are 1 and 2. Here is the word from
Volker Wichmann, the author of the module:
polygons, the table should look like this (tab-delimited example for two
polygons [just two simple rectangles]):
Where ID is an identifier unique to each polygon, and X and Y are
coordinates for each polygon vertex."
|12. And of course
you could have chosen LINES or POINTS in Panel 7 above, to create any type
of shape you want. So whenever the object location is defined by an
existing set of coordinates, you can easily use this method to create and
save the shapefile.