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 > 2.20 Land Stations

2.20 Removing "Land Stations" from a Marine Data Collection

  • Exercise Title:  Removing "Land Stations" from a Marine Data Collection

  • Abstract:  Very simple -- if not elegant-- method to eliminate data stations in any point shape that plot on land.  The original spreadsheet or ODV collection that provided the point shape is unchanged.

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

    • N/A

  • Required Software:

  • Other Resources: 

  • Author:  Murray Brown

  • Version:  8-19-2015

1.  Run Saga
2.  Set up the necessary data:
  • Load the World Borders polygon shape
  • Load a point shape, made from an ODV exported spreadsheet:  temp_from_osd_jas_0m_5m _liberia_wod_odv_saga made in the exercise Converting a Data Table to a Point Shape in Saga.  [Actually any point shape with stations on land will do.  "On land" means landward of the official coastline.]
  • Set POINTS = select the point shape
  • Set POLYGON = select World Borders

Click to the right of the ATTRIBUTES > NO ATTRIBUTES item to open a sub-menu.

4.  This table will appear:
  • Set ATTRIBUTE to any attribute in World Borders that certainly would not already be in the point shape, such as FIPS (US standard country name codes)
  • Click OK
5.  Now you can see the complete tool setup.  The last attribute, FIPS, is number zero in the list above; hence the value zero here
  • Click OK again
6.  Now you should see a new shape, with an extended name that indicates the source of the new field
7.  Right-click on the new shape and select ATTRIBUTES > SHOW. 
8.  The new shape is exactly like the old one, but it has a new attribute column named FIPS at the far right end (shown here). 

Only the points inside the polygons (i.e. on land) have any data values in FIPS, so these not-numbered features are at sea.  By chance, we are seeing a number of them at the top of the table.


9.  Right-click on any field name, and select SORT.  Sort the attribute table on the FIPS column, descending, so you see the on-land stations listed first (i.e. they have FIPS codes).
10.  Block all the records with FIPS codes
11.  Right click along the left column of the table, and select DELETE to remove them.  The new shape is now free of land stations (i.e. at-sea only).

NOTE:  If the number of deleted records is very large, then you must close and re-open the attribute table to see the result.

12.  If you want, you can delete the FIPS field completely, because it is no longer needed. 
  • Right click on the top of the table and select DELETE FIELDS.
  • Check FIPS

 Then click OK

13.  You can use ADD TO MAP to see the new shape.  Here the station symbols have been greatly reduced to make it easier to check along the coast.

To change the symbols, select SIZE > ATTRIBUTE > DEFAULT SIZE = 1 in the properties panel.

14.  Make sure to save the new shape with the filename temp_from_osd_jas_0m_5m _liberia_wod_odv_saga_noland (or something similar) in the folder PRODUCTS > SAGA > POINTS.