Converting a Data Table to a Points Shape in Saga

Marine Data Literacy

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5.1 Converting a Data Table to a Point Shape in Saga

Exercise Title: Converting a Data Table to a Point Shape in Saga
Abstract: In this exercise you will learn how to convert the data tables from ODV into "point shapes" in Saga. In the process, the tables will be inspected visually, and their histograms will be examined to find possible valid data ranges versus possible invalid data ranges (for temperatures in this example). Point shapes can be displayed as data products in their own right, but we intend to grid these data later.
Preliminary Reading (in OceanTeacher, unless otherwise indicated):
- Gridded Data
Required Software:
- Saga
Other Resources:
- data_from_osd_jfm_jas_0m_5m_3500m_4500m_liberia_wod_noheader.zip - Data tables (exported from ODV in Exporting Marine Data from ODV: Data Subsets); if you made them, then use your own files, otherwise put this file in PRODUCTS > ODV > SUBSETS and unzip it for use here
Author: Murray Brown
Version: 6-17-2014

1. Use any good ASCII editor, like ConTEXT, but never Excel, to remove the extra header lines from all of the above data subsets (the TXT files) and then save them. Currently there are eleven of these unwanted lines on all ODV-derived spreadsheets.

OPTION: You can add "noheader" to the filenames to help you remember these are edited versions, if you want.

2. Run Saga. Then use FILE > TABLE > LOAD to load all 4 of the "noheader" tables.

3. You should see these objects in Saga now. Right-click on the first table, and select SHOW.

4. It's always a good idea to visually inspect data before use.

Search the table to see what exported from ODV, and if there are any empty fields (there should be none).
Note that the date/time field is sometimes not complete or usable, a small problem.

You can close the table.

5. The first thing we need to do, is to convert this table to a point shape.

Select TOOLS > SHAPES-POINTS > CONVERT A TABLE TO POINTS

6. This window appears.

7. Make these choices:

SHAPES > POINTS > CREATE - Saga recalls the last job you did, and sometimes places the name of an existing file here.
For TABLE, drop down the menu and select the first of the 4 tables.

NOTE: The fifth object, the world borders shape, is listed here because it contains a table in its DBF file.

8. For X choose the LONGITUDE, and for Y choose the LATITUDE.

Then click OK.

NOTE: Leave the Z line untouched to convert the entire data table to a point shape.

9. This new object, a POINT SHAPE appears in the data tree of Saga.

10. Repeat this process with the other 3 tables. But make sure that the top line always says CREATE, when you run this module.

11. You should have these 4 point shapes, when finished.

12. Now, we should inspect the point shapes. Right-click on the first one, and select ADD TO MAP.

13. Select NEW, and OK.

14. Here are the points, displayed using default colors. The JFM points are on the left, and the 3500-4500 m products are on the bottom.

15. Select any one of the point shapes (in the left-hand menu) as the active object. In the properties panel for this object, make these selections:

DISPLAY > OUTLINE - Uncheck the box
COLORS > TYPE > Graduated color
COLORS > SCALING > 100 COLORS
COLORS > SCALING > PRESETS > Rainbow
COLORS > VALUE RANGE > ATTRIBUTE > Temperature

Then click SETTINGS > APPLY

16. Apply these settings to all four maps. Notice that the VALUE RANGE is not set above. Let Saga apply the natural range of each dataset, for now.

17. Here are all four maps of temperature values, organized as above. There's a lot of variation and many color palette issues to be solved here, as you can see from the variations.

There clearly is a cool area to the southwest in the deeper dataset.

18. Right-click on each point shape, and select SHOW HISTOGRAM to see that the distribution of values. Here are all 4 graphs showing that there are many "outlier" values that need to be evaluated.

Are they good, or should they be ignored in the analyses?

There appear to be 2 separate recommended ranges:

0-m Temp: 21-30 degrees C
4000-m Temp: 2.2-2.5 degrees C

Or something like that.

19. So you could change the value ranges to be limited to these extremes, and perhaps your figures would look better. It's your data and your call.

20. Now it's time to save your work. Right-click on the first of the point shapes, and select SAVE AS.

21. Navigate to the folder PRODUCTS > SAGA > VECTORS and save the point shape with the name data_from_osd_jfm_3500m_4500m_liberia_wod_odv_saga. Saga will actually save 4 files, with extensions SHP, DXF, SHX and MSHP (metadata for the shape).

22. In the same way, save all your new point shapes, using these filenames:

data_from_osd_jas_3500m_4500m_liberia_wod_odv_saga
data_from_osd_jfm_0m_5m_liberia_wod_odv_saga
data_from_osd_jas_0m_5m_liberia_wod_odv_saga

23. Now you have the point shapes needed for gridding. All GIS software goes through approximately these same steps, so the Saga model is good training that should prepare you for other systems.