I wanted to know how can I calculate land/sea area ratio in a portion of raster map (attached in the mail) Till now i had this raster map that i converted in to vector by the option raster>>conversion>>polygonise then i install the plugin CATOGRAM to draw grids. However i get no output.

. .I need to calculate it for the red portion represented in the attached image file covering my domain

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  • Welcome to GIS SE! As a new user be sure to take the Tour to learn about our focussed Q&A format. I recommend not thinking about GIS SE as being some sort of online GIS tutor. For your questions to be answered here they should as much as possible describe not just what you want to do, but precisely what you have tried and where you are stuck trying that. First I think you should decide which desktop GIS you are using. – PolyGeo Oct 27 '16 at 9:06
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    its a very important part of my research. I have tried a lot of things already Including "R" TabeleU and NCL i cannot get the answers for this there. I further tried a lo on QGIS but cannot figure it out. As a new user of QGIS i cannot describe it correctly, but i did try a lot @ployGeo – saurabh Oct 27 '16 at 9:10
  • I suspect that QGIS could do this. What precisely have you tried? Please use the edit button beneath your question to revise it with more details. – PolyGeo Oct 27 '16 at 9:14
  • You could perform a supervised classification with two training sets - land and water. What level of precision do you even need? It wouldn't take long to draw some rough polygons around the land to get the total land area and divide by the area of the red rectangle to get sea area. – DMusketeer Oct 27 '16 at 10:39
  • medium to low accuracy I do not require a very high accuracy @DMusketeer – saurabh Oct 27 '16 at 18:17

An easiest way to calculate the land/sea area is to do the following:

  • Download the shapefiles of administrative boundaries of Malaysia and Indonesia from gadm
  • Merge the two shapefiles together to have them in one shapefile
  • Create a new rectangle shapefile to clip the merged polygon shapefile
  • Use Union tool to union the clip polygon with the clipped administrative shapefile resulted from the previous step.
  • You will have a new shapefile that represents a land and a sea,and you can calculate their areas. But before doing that, don't forget to project the output from the last step to a suitable projection in meter like (UTM) before calculating the area.
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  • This is a good idea but probably starting with more complex data than you need. Google a world countries shapefile. – DMusketeer Oct 28 '16 at 14:11

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