Wednesday, February 21, 2018

QGIS STEPS #2 - SEARCH || QUERY IN QGIS

Selection by attribute
Open a layer (e.g. Nepal_84.shp)
Open attribute table by right clicking on layer or toolbar
Click on e toolbar (next to select toolbar)
Click on “select by expression”
In the opened window use the following expression to search
Select all district starting with k
DISTRICT is a field name
“DISTRICT” like ‘K%’
Select all district starting with k and ending with i
“DISTRICT” like ‘K%i’
Select district starting with L and with 7 character more
(‘L_______’ = L 7 underscore
“DISTRICT” like ‘L_______’
Select a district by exact name
“DISTRICT” = ‘Kathmandu’
Select a district with area less than 700 sq.km
(Area is a field name containing areas in sq. km)
(to search area in sq. ft, create a new field “Area_sqft” and write formula to change sqkm to sqft and search as
“Area_sqft” < 200 รจ search area less than 200 sqft
“Area” < 700
Select area greater than 700 sqkm and less than 2000 sqkm
“Area” > 700 and “Area” < 2000
Select district starting with K or L
“DISTRICT” like ‘K%’ or “DISTRICT” like ‘L%’


Watershed/Catchment area in QGIS

1.       Load pore_point vector and Kathmandu_dem raster file in Qgis

2.       Go to processing menu bar>toolbox

3.       Search for pit remove which is under taudem>basic grid                  analysis tools>pit remove >open it and give                                      “Kathmandu_dem” as input layer>click run>”pit removed                 elevation gird” raster file is created

4.       Now click on D8 flow direction under taudem>basic grid                  analysis tools>D8 flow direction>open it and give the input              layer as “pit removed elevation gird”>click run>a raster file is          created whose name is “D8 flow direction grid”

5.       Right click on the “D8 flow direction grid”>go to properties>          change max to 8 and min to 1

6.       To find flow accumulation we use taudem>basic grid analysis          tools>D8 contributing area>give input as “D8 flow direction            grid” and output shape file as “pore_point”>click run> “D8              contributing area grid” named raster is created

7.       Go to “stream network analysis tools”> “stream definition by          threshold”>input layer as > “D8 contributing area grid” and              threshold value as 100000>”stream raster grid” is                              created>change its property as : max=1 and min=0

8.       Generate the catchment area by clicking on “Stream reach and          watershed” under “stream network analysis tools” Input file as        >”pit removed elevation gird”; “D8 flow direction grid”; “D8          contributing area grid”; ”stream raster grid” and output shape          file as “pore_point”>click run
9.       Convert the “watershed grid” raster file to vector as>                        raster>conversion>raster to vector
      This is how catchment area for a pore point is created.





CALCULATING VOLUME OF RESERVOIR

Input: Watershed polygon
                DEM_org
1.    Creating polygon of watershed as previous but here we        use model to create watershed polygon

2.   Then use raster calculator to subtract the elevation of             DEM  file from 1310m. This will give all elevation               above 1310m negative and below 1310m positive
   Raster menu—raster calculator then Rast1 is created

3.    Reclassify the rast1 file as : put all values less than 0 as 0     and all other values as 1 (between 0 to 6000m)
   To get reclassify tool: processing—toolbox--search for         reclassify value—input Rast1 layer and select simple           table in method then go to table and enter value as
                                 Min        Max       value
                                -2000        0           0                             
                                0              6000       1
                                Uncheck “replace other values”
                                Click run
                                Change property of “reclassified grid” as                                            max=1 and min=0

4.     Convert “reclassified grid” to polygon as
Raster—conversion—raster to vector and name it polygon1

5.     Right click on polygon1—go to attribute table and delete      all values of DN except value “1”

6.    Clip polygon1 with watershed and name polygon2
   Vector menu—geoprocessing-clip and name it polygon2

7.    Calculate area Km2 of polygon2 from field calculator of       attribute table

8.    Go to raster-zonal statistics-zonal statistics—click ok

9.    Multiply area_km2 and “_mean” in field calculator as
   “area_km2” * “_mean” /1000 (since “_mean” is  in meter)

10.  vector -> analysis -> basic statistics to get reservoir               volume

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