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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