Using map algebra in GRASS - Revision history

Mabel Chua: /* Subtracting raster to fit into study area */ accesssing r.mapcalc

2021-10-02T03:48:13Z

Subtracting raster to fit into study area: accesssing r.mapcalc

2021-10-02T03:45:03Z

Conclusion

2021-10-02T03:37:47Z

added link to dataset

2021-10-02T03:32:51Z

2021-10-02T03:26:48Z

rules of map algebra

2021-10-02T02:49:49Z

added example photo

2021-10-02T02:44:24Z

Undo revision 17411 by Mabel Chua (talk)

2021-10-02T02:43:25Z

added example photo

2021-10-02T02:32:02Z

expanded the intro/what is map algebra?

2021-10-02T01:47:02Z

expanded on the introduction

@@ Line 52: / Line 52: @@
 To start, I have two DEMs one that is smaller than the other as you can see in the picture.
 [[File:Ex 1 1.png|500px|thumb|center]]
+In GRASS you can access the raster map calculator by going to the raster menu, from the dropdown menu click raster map calculator and then make sure that you are accessing raster map calculator (also shown as r.mapcalc)
 In order to subtract the excess from the large DEM to make it the same size as the as the smaller DEM which for this example will be our study area I used the command

@@ Line 87: / Line 87: @@
 == Conclusion ==
-r.mapcalc is a very useful skill to know in GRASS as GRASS is used to do a lot of analysis of raster data and in order to do that you need to know how to manipulate raster data. I made this tutorial because I had trouble with GRASS and hopefully this will help someone else have not as much trouble.
+r.mapcalc is a very useful skill to know in GRASS as GRASS is used to do a lot of analysis of raster data and in order to do that you need to know how to manipulate raster data. I made this tutorial because I had trouble with GRASS and hopefully this will help someone else have not as much trouble. Knowing how to operate map algebra in GRASS is a powerful tool as it gives access to various tools and operations within GRASS to analyze raster data.
 == References ==

@@ Line 49: / Line 49: @@
 ==== Subtracting raster to fit into study area ====
-For this first example the DEMs (Digital Elevation Model) that are used can be obtained from the Government of Canada website. However, this example will work with any raster file in the same situation.
+For this first example the DEMs (Digital Elevation Model) that are used can be obtained from the Government of Canada website.https://open.canada.ca/data/en/dataset/6640b919-abbe-405b-a5f6-d4d808cbbcec However, this example will work with any raster file in the same situation.
 To start, I have two DEMs one that is smaller than the other as you can see in the picture.
 [[File:Ex 1 1.png|500px|thumb|center]]

@@ Line 32: / Line 32: @@
 # Then you add in your operator and variables for example: !fileA which means not file A
  r.mapcalc newFile= !fileA
 this will give you a raster that is the inverse of file A
+Note, that the spacing in the above command is purposeful, to separate the different aspects of the expression in order to produce the needed operation.
 The other way to do mapcalc is without the command line using the map calculator built into the GUI, which is accessed using the button shown below.

@@ Line 18: / Line 18: @@
 this command basically tells the program that for the new raster that you are creating, if raster B(the study area) is in that location then put raster A(the raster you are cutting) there, meaning that the end result should be whatever you are measuring in raster A in just the area of raster B.
+Some basic rules are important to remember when executing map algebra because no matter how basic or complex the command is, GRASS must be able to coherently read it, to execute the command. It is important to remember that map algebra is case sensitive, so knowing when to capitalize and when not to is crucial. Additionally, parentheses are often used to indicate the tool parameters being applied to the input dataset.
 ==== What can you do with Map Algebra? ====

@@ Line 49: / Line 49: @@
 "r.mapcalc --overwrite expression=DEM = DEM@elmin if( DEM@elmin, DEM__2_@elmin)"  (elmin is the mapset name just happens that is my username on my computer).
+[[File:Tutorial revision (example 1).png|thumb|center|subtracting raster in command line]]
 the result as you can see in this picture is that both rasters fit in the study area.

@@ Line 11: / Line 11: @@
 ==== What is Map Algebra? ====
-Map algebra is a way in GIS to perform arithmetic on raster layers in maps. The way that it works is that it will take two raster layers and it performs an operation on them based on what you tell it to do. For example, if you wanted to cut a raster image down to the study area of another you could do that by using the following command:
+Map algebra is a way in GIS to perform arithmetic on raster layers in maps. The way that it works is that it will take two raster layers and it performs an operation on them based on what you tell it to do. The basics to know before starting map algebra is to know that map algebra is like a language and in order to communicate the statement to GRASS, the program follows a rules to effectively read the command given. Using r.mapcalc, the expression can either be typed out in the command line or use the preset buttons on top to assemble the command.
+One way access the r.mapcalc is through going to the Raster menu, then to Raster map calculator, and then the dropdown menu will have the option for Raster map calculator (r.mapcalc).
+For example, if you wanted to cut a raster image down to the study area of another you could do that by using the following command:
  r.mapcalc --overwrite rasterAnew=”if(rasterB, rasterA)”

@@ Line 5: / Line 5: @@
 == Introduction ==
-GRASS is geographic information system (GIS) software that is used for geospatial data management and analysis. If you find yourself working on raster data specifically in GRASS it may be very useful to know how to use map algebra in order to manipulate the data so that you can properly analyze it. Also it is important to know how to use this tool using the command line instead of the graphical user interface (GUI) and so to that end for this whole page all of the examples will be done using the command line commands.
+GRASS is geographic information system (GIS) software that is used for geospatial data management and analysis. If you find yourself working on raster data specifically in GRASS it may be very useful to know how to use map algebra in order to manipulate the data so that you can properly analyze it. The advantages to understanding map algebra in GRASS is that it allows for manipulation of raster layers in a program that is focused on data processing and analysis. Also it is important to know how to use this tool using the command line instead of the graphical user interface (GUI) and so to that end for this whole page all of the examples will be done using the command line commands.
 == Map algebra ==