Showing posts with label GIS. Show all posts
Showing posts with label GIS. Show all posts

Extracting Raster Values from Points in R and GRASS

Monday, July 26, 2010

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages.

The Code

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values.

Posted by toddjobe Labels: , , at 3:43 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS

Saturday, July 24, 2010

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 6:56 AM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 6:51 AM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 6:49 AM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 6:41 AM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is =ExtractValuesToPoints= available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS. ** Extract Values to Points in R This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS. *** Data required For the purpose of this exercise. All the data must be have the same spatial projection. - [[file:data/gr.asc][=gr.asc=]] :: an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post. - [[file:data/pt.zip][=pt.shp=]] :: a point shapefile. You also need the =maptools= and =sp= packages in R. *** The Code #+begin_src R -t :results output :colnames t library(maptools) # autoloads sp gr <- readAsciiGrid("data/gr.asc") pt <- readShapePoints("data/pt.shp") overlay(gr, pt) #+end_src #+results: #+begin_example coordinates gr.asc 1497 (569292, 1224170) 6094.6080 539 (567718, 1227840) 7964.5331 1023 (564565, 1225810) -293.6599 663 (562462, 1227260) -5351.7297 69 (563675, 1229710) -2923.9394 716 (563062, 1227180) -4241.8255 339 (567636, 1228780) 4781.6488 509 (561722, 1227870) -3958.7312 805 (560981, 1226690) 139.9091 155 (560884, 1229220) -2719.7353 #+end_example That is it. Fast, and easy. ** Extracting Values in GRASS Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into. *** Data Required - =gr= : A GRASS grid - =pt= : A GRASS point dataset *** The Code The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. =varchar(10)= string of length 10, =double precision= floating point numbers, =int= integers). Then, fill the column with the raster values. #+begin_src sh :results: silent v.db.addcol map=pt columns="grval double precision" v.what.rast vector=pt raster=gr column=grval #+end_src

Posted by toddjobe Labels: , , at 6:40 AM 0 comments

 

Extracting Raster Values from Points in R and GRASS

Friday, July 23, 2010

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is =ExtractValuesToPoints= available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS. ** Extract Values to Points in R This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS. *** Data required For the purpose of this exercise. All the data must be have the same spatial projection. - [[file:data/gr.asc][=gr.asc=]] :: an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post. - [[file:data/pt.zip][=pt.shp=]] :: a point shapefile. You also need the =maptools= and =sp= packages in R. *** The Code #+begin_src R -t -w 66 :results output :colnames t library(maptools) # autoloads sp gr <- readAsciiGrid("data/gr.asc") pt <- readShapePoints("data/pt.shp") overlay(gr, pt) #+end_src #+results: #+begin_example coordinates gr.asc 1497 (569292, 1224170) 6094.6080 539 (567718, 1227840) 7964.5331 1023 (564565, 1225810) -293.6599 663 (562462, 1227260) -5351.7297 69 (563675, 1229710) -2923.9394 716 (563062, 1227180) -4241.8255 339 (567636, 1228780) 4781.6488 509 (561722, 1227870) -3958.7312 805 (560981, 1226690) 139.9091 155 (560884, 1229220) -2719.7353 #+end_example That is it. Fast, and easy. ** Extracting Values in GRASS Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into. *** Data Required - =gr= : A GRASS grid - =pt= : A GRASS point dataset *** The Code The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. =varchar(10)= string of length 10, =double precision= floating point numbers, =int= integers). Then, fill the column with the raster values. #+begin_src sh :results: silent v.db.addcol map=pt columns="grval double precision" v.what.rast vector=pt raster=gr column=grval #+end_src

Posted by toddjobe Labels: , , at 3:30 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 3:29 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code 

library(maptools) # autoloads sp
gr <- readAsciiGrid("data/gr.asc")
pt <- readShapePoints("data/pt.shp")
overlay(gr, pt)

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 3:25 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotely sensed environmental variable at a point location. For instance we may wish to extract the elevation of a field plot from a digital elevation model. The elevation data is a raster (i.e. grid) and the plots are a point shapefile (or a simple text file of X, Y locations). The command for doing this in ArcGIS is ExtractValuesToPoints available in the Spatial Analyst package. Situations may arise where ArcGIS is not the most efficient way of extracting these values. So, here, I provide a brief overview of how to extract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires more statistical sophistication than is available in ArcGIS. As a result, I have completely switched to doing the extraction in R. I known I am going to end in R eventually, and it is easier to automate than writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have the same spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGIS binary grid if you know how to use RGDAL. That perhaps will be another post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code 

library(maptools) # autoloads sp
gr <- readAsciiGrid("data/gr.asc")
pt <- readShapePoints("data/pt.shp")
overlay(gr, pt)

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, but it takes a little bit more planning in that you have to explicitly create the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in the point dataset with the right data type (i.e. varchar(10) string of length 10, double precision floating point numbers, int integers). Then, fill the column with the raster values. 

v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 3:07 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS


A common task in GIS analysis is to extract the value of a remotely
sensed environmental variable at a point location. For instance we
may wish to extract the elevation of a field plot from a digital
elevation model. The elevation data is a raster (i.e. grid) and the
plots are a point shapefile (or a simple text file of X, Y
locations). The command for doing this in ArcGIS is
ExtractValuesToPoints available in the Spatial Analyst package.
Situations may arise where ArcGIS is not the most efficient way of
extracting these values. So, here, I provide a brief overview of how to
extract raster values to points in R and GRASS.


Extract Values to Points in R



This is strikingly easy is R. My work usually requires more
statistical sophistication than is available in ArcGIS. As a
result, I have completely switched to doing the extraction in R. I
known I am going to end in R eventually, and it is easier to automate
than writing a long python script in ArcGIS.


Data required



For the purpose of this exercise. All the data must be have the
same spatial projection.



gr.asc

an ESRI ASCII grid. This could also be an ArcGIS
binary grid if you know how to use RGDAL. That perhaps will be
another post.

pt.shp

a point shapefile.


You also need the maptools and sp packages in R.


The Code 


library(maptools) # autoloads sp
gr <- readAsciiGrid("data/gr.asc")
pt <- readShapePoints("data/pt.shp")
overlay(gr, pt)


That is it. Fast, and easy.


Extracting Values in GRASS



Extracting raster values in GRASS is somewhat faster than in R, but
it takes a little bit more planning in that you have to explicitly
create the column that the raster values will go into.


Data Required




  • gr : A GRASS grid


  • pt : A GRASS point dataset


The Code


The basic flow of this is that you create an empty column in the
point dataset with the right data type (i.e. varchar(10) string
of length 10, double precision floating point numbers, int
integers). Then, fill the column with the raster values.


v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 3:03 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS

A common task in GIS analysis is to extract the value of a remotelysensed environmental variable at a point location. For instance wemay wish to extract the elevation of a field plot from a digitalelevation model. The elevation data is a raster (i.e. grid) and theplots are a point shapefile (or a simple text file of X, Ylocations). The command for doing this in ArcGIS isExtractValuesToPoints available in the Spatial Analyst package.Situations may arise where ArcGIS is not the most efficient way ofextracting these values. So, here, I provide a brief overview of how toextract raster values to points in R and GRASS.

Extract Values to Points in R

This is strikingly easy is R. My work usually requires morestatistical sophistication than is available in ArcGIS. As aresult, I have completely switched to doing the extraction in R. Iknown I am going to end in R eventually, and it is easier to automatethan writing a long python script in ArcGIS.

Data required

For the purpose of this exercise. All the data must be have thesame spatial projection.

gr.asc
an ESRI ASCII grid. This could also be an ArcGISbinary grid if you know how to use RGDAL. That perhaps will beanother post.
pt.shp
a point shapefile.

You also need the maptools and sp packages in R.

The Code 

library(maptools) # autoloads spgr <- readAsciiGrid("data/gr.asc")pt <- readShapePoints("data/pt.shp")overlay(gr, pt)

That is it. Fast, and easy.

Extracting Values in GRASS

Extracting raster values in GRASS is somewhat faster than in R, butit takes a little bit more planning in that you have to explicitlycreate the column that the raster values will go into.

Data Required

  • gr : A GRASS grid
  • pt : A GRASS point dataset

The Code

The basic flow of this is that you create an empty column in thepoint dataset with the right data type (i.e. varchar(10) stringof length 10, double precision floating point numbers, intintegers). Then, fill the column with the raster values.

v.db.addcol map=pt columns="grval double precision"v.what.rast vector=pt raster=gr column=grval

Posted by toddjobe Labels: , , at 3:02 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS


A common task in GIS analysis is to extract the value of a remotely
sensed environmental variable at a point location. For instance we
may wish to extract the elevation of a field plot from a digital
elevation model. The elevation data is a raster (i.e. grid) and the
plots are a point shapefile (or a simple text file of X, Y
locations). The command for doing this in ArcGIS is
ExtractValuesToPoints available in the Spatial Analyst package.
Situations may arise where ArcGIS is not the most efficient way of
extracting these values. So, here, I provide a brief overview of how to
extract raster values to points in R and GRASS.




Extract Values to Points in R






This is strikingly easy is R. My work usually requires more
statistical sophistication than is available in ArcGIS. As a
result, I have completely switched to doing the extraction in R. I
known I am going to end in R eventually, and it is easier to automate
than writing a long python script in ArcGIS.






Data required






For the purpose of this exercise. All the data must be have the
same spatial projection.



gr.asc

an ESRI ASCII grid. This could also be an ArcGIS
binary grid if you know how to use RGDAL. That perhaps will be
another post.

pt.shp

a point shapefile.




You also need the maptools and sp packages in R.







The Code 







library(maptools) # autoloads sp
gr <- readAsciiGrid("data/gr.asc")
pt <- readShapePoints("data/pt.shp")
overlay(gr, pt)






That is it. Fast, and easy.








Extracting Values in GRASS






Extracting raster values in GRASS is somewhat faster than in R, but
it takes a little bit more planning in that you have to explicitly
create the column that the raster values will go into.






Data Required






  • gr : A GRASS grid


  • pt : A GRASS point dataset








The Code




The basic flow of this is that you create an empty column in the
point dataset with the right data type (i.e. varchar(10) string
of length 10, double precision floating point numbers, int
integers). Then, fill the column with the raster values.





v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval






Posted by toddjobe Labels: , , at 3:00 PM 0 comments

 

Extracting Raster Values from Points in R and GRASS


A common task in GIS analysis is to extract the value of a remotely
sensed environmental variable at a point location. For instance we
may wish to extract the elevation of a field plot from a digital
elevation model. The elevation data is a raster (i.e. grid) and the
plots are a point shapefile (or a simple text file of X, Y
locations). The command for doing this in ArcGIS is
ExtractValuesToPoints available in the Spatial Analyst package.
Situations may arise where ArcGIS is not the most efficient way of
extracting these values. So, here, I provide a brief overview of how to
extract raster values to points in R and GRASS.




Extract Values to Points in R






This is strikingly easy is R. My work usually requires more
statistical sophistication than is available in ArcGIS. As a
result, I have completely switched to doing the extraction in R. I
known I am going to end in R eventually, and it is easier to automate
than writing a long python script in ArcGIS.






Data required






For the purpose of this exercise. All the data must be have the
same spatial projection.



gr.asc

an ESRI ASCII grid. This could also be an ArcGIS
binary grid if you know how to use RGDAL. That perhaps will be
another post.

pt.shp

a point shapefile.




You also need the maptools and sp packages in R.







The Code 







library(maptools) # autoloads sp
gr <- readAsciiGrid("data/gr.asc")
pt <- readShapePoints("data/pt.shp")
overlay(gr, pt)






That is it. Fast, and easy.








Extracting Values in GRASS






Extracting raster values in GRASS is somewhat faster than in R, but
it takes a little bit more planning in that you have to explicitly
create the column that the raster values will go into.






Data Required






  • gr : A GRASS grid


  • pt : A GRASS point dataset








The Code




The basic flow of this is that you create an empty column in the
point dataset with the right data type (i.e. varchar(10) string
of length 10, double precision floating point numbers, int
integers). Then, fill the column with the raster values.





v.db.addcol map=pt columns="grval double precision"
v.what.rast vector=pt raster=gr column=grval






Posted by toddjobe Labels: , , at 2:58 PM 0 comments