Marine Data Literacy 2.0

Providing instruction for managing, converting, analyzing and displaying oceanographic station data, marine meteorological data, GIS-compatible marine and coastal data or model simulations, and mapped remote sensing imagery

 

 

 

 

Home > 2. Marine GIS > 2.6 Google Digitizing

2.26 Digitizing High-Resolution Coastlines in Google Earth

  • Exercise Title:  Digitizing High-Resolution Coastlines in Google Earth

  • Abstract:  This method represents a huge advance in the easy digitizing of physical features, which may exist only in very coarse form in existing base maps.  Currently many global- or continent-scale databases are barely as good as 0.01-degree resolution (i.e. about 1 km), but with this method you can get very good vector shapes down to 0.00001-degree resolution (i.e. individual palm trees!).  The method is entirely dependent on the georeferencing of the base images published in Google Earth, so you should experiment with match-ups between products made here and other vector files of known quality (e.g. World Vector Shoreline, Global Self-Consistent Hierarchical High Resolution Geography Database) to insure the position accuracies of your own final products.  [There is a tool in Saga (SHAPES-TOOLS > CREATE NEW SHAPES LAYER) that can make shapes in a similar way.  But the ready availability of global, high-resolution imagery in Google Earth makes this exercise the preferred method.]

  • Preliminary Reading (in OceanTeacher, unless otherwise indicated):

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  8-4-2014

1.  Run Google Earth (GE)
2.  Enter the town names you see here and click on SEARCH
3.  The GE globe will rotate and focus in on this very small area of the Liberian Coast.  The resolution is so good that individual (palm?) trees are visible.
4.  You can add this location map to the list of TEMPORARY PLACES by simply dragging the label down from the upper panel to just below the TEMPORARY PLACES object.
5.  One way to digitize the coast is to mark it out with a PATH.  Select ADD > PATH.
6.  This small window opens.  You must keep it open to do the digitizing.  You can set the visual properties of the operation here, however you wish.
7.  Examine the coastline carefully, and decide what color signature you want to identify as the actual line.  If this is your first digitizing exercise, then you'll be surprised to learn that it is a very difficult job:  the colors are not what you might expect along the shore, i.e. blue and green.  Here's a very rough table of some possibilities:
Area General Depths or Heights (very rough estimates) Possible Colors/Materials
Open Water deeper than -10 m
  • Blue - clean marine waters
  • Blue/Green - marine waters with coastal input
  • Green/Brown - coastal waters; river influenced
Nearshore -10 to -0 m
  • All of the above for Open Water
  • White/Tan - clear water over sand/sediment
  • White - breaking waves
  • Green - Seagrass
Beach,  Shore, Intertidal -3 to +3 m
  • White - typical sandy beach
  • Other colors - black (rocks or Hawaii beach sand), green (maritime forest/marsh/no beach), etc.
Upper Shore >3 m
  • Green - forest, wetlands
  • White/Tan - coastal wasteland, extended dunes
8.  When you have made your choice, then follow the "coast" with your cursor and left-click on specific points that define outward and inward turns.
  • Left-click - Set a point or move a point
  • Right-click - Deletes a point
9.  If you change your mind about a point that appears to be wrong....
10. ... you can simply move it to a better location, with your cursor.
11.  In this way, move your way through the whole area.  Here we have moved straight across the mouth of the huge estuary, because there is an obvious white/tan beach across it.
12.  If you click on OK on the ADD panel, the red icons turn into a new line shape (barely visible here in white).
13.  You can adjust the new shapes properties, by right-clicking on it and selecting PROPERTIES.
14.  Here a wider (2.0 units) red line has been selected.  Click OK to effect these choices.
15.  Here you can see the new coastline.
16.  Now we need to convert the new coastline to a file, for use in other programs.
17.  Right-click on the new shape, and select SAVE PLACE AS.
18.  Navigate to the folder PRODUCTS > GOOGLE EARTH and save the file with the name coastline_rocktown_bigtown_liberia_landsat_google_earth.kmz
19.  Before we move on to work with this new KMZ file in Saga, we need to see one final thing you can do:  edit the file in GE.
20.  To edit a file in GE, right-click on it and select PROPERTIES.
21.  This returns you to the same view of the file you saw while digitizing it.  Here you can continue adding, deleting or moving points, as above.

This is the sort of change you could make, if -- for example -- you wanted to show the mouth of the estuary.

22.  This completes the digitizing.  Now what can we do with the files?
23.  You can always simply open the KMZ file, just by double-clicking on it in Windows Explorer.  KMZ files automatically open in GE, and you will immediately see the same map as in Panel 15 above.
24.  But how to work with the file in Saga?  Rename the KMZ file from *.KMZ to *.ZIP (yes Windows will squawk, but it's perfectly OK)
  • coastline_rocktown_bigtown_liberia_landsat_google_earth.KMZ --> coastline_rocktown_bigtown_liberia_landsat_google_earth.ZIP

Unzip the ZIP file in place to produce a KML file named doc.KML in a folder of the same name as the file.  Find it, and make this name change:

  • doc.KML --> coastline_rocktown_bigtown_liberia_landsat_google_earth.KML
25.  Run Saga.
26.  Select TOOLS > IMPORT/EXPORT-GDAL/OGR > IMPORT VECTOR DATA.

For FILES, find and select the KML file you just renamed and unzipped.

Then click OK.

27.  Here is the new line shape, displayed on a Saga map.
28.  If you need to, you can select the shape with the SAGA action tool (the arrow), and use right-click EDIT SELECTED SHAPE to use the same controls you saw for GE in Panel 8, above.  If you make changes, make sure to save the new shape over the old one.
29.  Now, just for a reality check, we've compared the new coastline with other ones:
  • Dotted Line - New shape created in this exercise
  • Solid Line - World Vector Shoreline (WVS) extract from GEBCO (see link above)
  • Green Polygon - GSHHG land polygon published in GADM (see link above for more information)

The mismatch between the new line and the WVS is surprising, but the WVS is quite old now.

30.  Make sure to save your new SHP file in PRODUCTS > SAGA > VECTORS with an appropriate name