Geo-processing

Exercise 4 Geo-processing

4.1  Dissolve
Dissolve is another tool used to trim away unnecessary detail that is not relevant to the scope of the project. Data can be simplified by dissolving several features in a layer into one. We use Dissolve when we want to aggregate features based on a specified attribute. Dissolve used to aggregate/group. The feature class based on their attribute data i.e. you can group uniform /homogeneous/ attribute data in to one single data.

To dissolve multiple features into one.
1. click the ArcToolbox
2. Data Management Tools
3. Generalization
4. Dissolve.

Ø  The Dissolve tool dialog box opens.
In the dissolve dialog box,

Ø  select the “Input Features” from the drop-down menu or browse to the feature.

Ø   specify a name and location for the “Output Feature Class”.

Ø  move to the ‘Dissolve_Field(s)” option. From the list of attributes select one or more on which to dissolve.

Ø  Once selected, click the ‘OK” button. The output is automatically added to the map display.

Exercise 4.1:

•         Dissolve Ethiopia_woreda by Zone,

•         Dissolve Ethiopia_woreda by Zone,

•          How many banks are in each region (in which regions highest concentration?

•         Divide the total number of banks to the total population 

4.2  Merge
The merge function helps to merge two and more feature class in to one feature class. Example when you transform a land use map of one community watershed, the procedure is for transform is transform each land use GPS point in separate file so after finishing separate data then you can merge to get one land use map or that specific community watershed.

1. Click the plus sign in front of the Data Management Tools
2. Click the plus sign in front or general
3. Double click on the merge function
4. Select the input feature class
5. Change the folder and file the output data
6. Click ok

Exercise 4.2: 

F Region in Ethiopia are given in exercise file as separate feature layer, Merge them all together  and create one feature layer 

F  from your ethio_GIS data you found problem in calculating area, pop Density for Amhara (because Lake Tana  as different feature) the Solution is merging

F Open Ethio-zone and merge zones in the same region together

Try the same exercise data but with union tool. What difference do you observe?

Overlaying operations :

An overlay operation is also one of the most common spatial analysis tools. Overlays allow you to identify areas where features in Iwo layers overlap. A new data set may be created based on these overlaps
4.3  Clip

clip is used when we want to cut out a piece of one layer using one or more of the polygons in another layer in order to work with only the data for an area of interest.
The layer that will have its features clipped can contain points, lines, or polygons. If we only want a subset or the polygon in this layer to be clipped we can select them using any of ArcMap”s  feature selection tools, before we start the Geoprocessing wizard”. The layer on which the clip will be based must contain polygon features.

1. Go to the Toolbox
2. Analysis tool
3. Extract
4. clip this opens open the clip window

§  under “Input Feature” select the map to be clipped

§  under “Clip Feature” select the map that will be used to perform the clip

§  under “Output Feature” indicate where to save the new layer and give it a name
The attributes of the features in the output layer will be the same as those of the feature in the layer being clipped.

Ø  Exercise 4.3:

Ø  Clip the rivers, roads and towns found in SNNPR

Ø  Clip towns in Amhara region which has a total population > 200000

o    

4.4 Intersect
we use intersect when we want to overlay a layer with the polygons in another layer so that the resulting output layer

a.       has the combined attribute data of the features from the two inputs, and

b.      Only contains features that fall within the spatial extent of the overlay polygons.
In this way, we can find those features that overlap and ,stamp’ the attributes of the overlay polygons in the second layer onto the features in the first layer.

we can intersect polygons with polygons and lines with polygons. We can specify that the intersection be limited to the currently selected set of features in either or both of the two inputs. If we only want a subset of the features in either or both layers to be used in the process, we can select these features using any of ArcMap”s feature selection tools.

To perform an intersect overlay,
I. go to “Analysis Tools” in AreToolbox and
2. click on Overlay”.
3. From the list or Overlay tools double-click on Intersect.
4. You arc prompted to select the input features for intersection and the output feature class
5. Click the ok button. The result is automatically displayed in ArcMap.

                                                                                                                                                           

Exercise 4.4:

Ø  To keep the GRD /from siltation soil conservation should be done. So, in which  woredas the government of Oromia region should practiced soil conservation in relation to the project. 

Ø  I.e  what is the intersection between Oromia and Abay Basin

Ø  How many  woredas of Oromia have direct  role in protecting the dam from siltation

Ø  How much percent of Abay Basin found in Oromia

4.5  Union
we use Union when we want to overlay two polygon layers so that the resulting output layer

a.       has the combined attribute data of the polygons in the two inputs, and

b.       Contains all the polygons from the inputs, whether or not they overlap.

In this way, we can produce a new layer combining the features and attributes of two polygon layers.
The output from this Geoprocessing operation will be in the same coordinate system as the data frame, irrespective of the coordinate system of the data source of the input layers (exception:
when saving the output from this operation as a feature class in an existing feature data set).

To perform a union overlay.
I. go to ArcToolbox's “Analysis Tools”.
2. select “Overlay”,
3. and then click “Union".

4. Specify the input layers to the union and a name for the output data set.
5. click ok, the result will be added to the ArcMap display if Arc Toolbox was used through ArcMap.

 

Exercise 4.5 :

 you want to produce the earliest map of Ethiopia but, the data you have is the recent one, Ethiopia and Eritrea are two different sovereign country. Can you produce that?

4.6 Erase

Contrast Update with Erase. Erase removes the overlapping features from the input layer. In this example, remove the Lakes areas completely from the Soils layer.

To perform an Erase,
I. go to “Analysis Tools” in AreToolbox and
2. click on Overlay”.
3. From the list or Overlay tools double-click on Erase.
4. You arc prompted to select the input features for intersection and the output feature class
5. Click the ok button. The result is automatically displayed in ArcMap.

Exercise 4.6:

 Erase areas covered by the lakes from the soil Layer

Exercise5. 6: mask annual precipitation of SNNPR from Ethiopia annual precipitation, where

4.7  Masking

Extract by mask function /clip Raster data.

1. Click the Spatial Analyst Tools
2. Click on Extraction
3. Double click on extract by mask function
4. Select raster input map
5. Select raster or feature mask data
6. Change the name or the folder and file name of your output data
7. Click ok