Vectorization in GIS

In the world of Geographic Information Systems (GIS), rasters and vectors are two commonly used forms of data representation. Rasters are digital images composed of pixels, each representing a single geographic location, while vectors are composed of discrete geometric shapes, such as points, lines, and polygons, representing geographic features.

The process of converting rasters into vectors is known as vectorization. This process is useful for a variety of GIS applications, such as land use mapping, hydrological analysis, and terrain modeling.

In this article, we will explore the basics of vectorization and the various methods and tools available for converting rasters into vectors. We will cover topics such as polygonization, conversion of rasters into points, and the processing of multiband rasters.

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A brief introduction to Raster Classification

The classification process involves grouping the values in a raster into a smaller number of categories or classes based on certain criteria. The goal of classification is to simplify the raster data and to make it easier to analyze, visualize, and work with.

The process of classification involves the following steps:

1. Determine the number of classes to be used.
2. Define the criteria for grouping the values into classes. This could be based on ranges of values, statistical clustering, or other methods.
3. Assign each cell in the raster to a class based on the defined criteria.
4. Create a new raster with the classified values, where each cell has a value representing its class.

The specific method used for classification will depend on the type of data in the raster and the goal of the classification. Common methods include:

• Equal interval classification
• Quantile classification
• Natural breaks (Jenks) classification
• K-means classification

A raster does not necessarily need to be classified before it is vectorized, but it is often beneficial to do so. Classifying a raster involves grouping the values in the raster into a smaller number of categories, or classes, based on certain criteria.

This can be useful in polygonizing a raster because it reduces the number of polygon features that are generated, making the vector layer more manageable and easier to work with.

For example, if the raster represents elevation data, you may classify the raster into a few classes, such as low, medium, and high elevations, rather than having separate polygon features for every unique elevation value.

However, if the raster data already has a limited number of unique values, then classifying the raster may not be necessary before polygonizing it.

Vectorizing multiband rasters in GIS

Multiband rasters typically need to be processed or analyzed before they can be converted into vectors. The specific processing steps required will depend on the data and use case, but in many cases, converting the multiband raster into one or more single-band rasters can be a necessary step.

Single-band rasters contain data for a single attribute or variable, such as elevation, temperature, or reflectance, while multiband rasters contain data for multiple attributes or variables, each represented by a separate band.

In order to convert a multiband raster into vectors, you typically need to extract the information from one or more of the bands that you want to represent as vectors.

For example, if you have a multiband raster with red, green, and blue bands, you might want to convert just the blue band into a single-band raster, and then use the “Contour” or “Isoband” tool to generate the vectors based on that data.

Converting raster to polygons in GIS

Polygonizing a raster means converting the raster data, which is represented as a grid of cells with values or classes, into a vector format, where each cell is represented as a polygon feature with attributes. The result is a vector layer of polygons that represent the different classes or values in the original raster.

In this process, each cell in the raster is treated as a separate polygon feature, with its attributes being the value or class assigned to that cell in the original raster. This conversion process is useful in many GIS applications, as vector data is often easier to analyze, visualize, and manipulate compared to raster data.

Polygonizing a raster is also called “raster to vector conversion,” “raster vectorization,” or “raster to polygon conversion.”

to polygonize a raster, you can use GIS software such as QGIS or ArcGIS. The process involves converting a raster dataset into vector polygon features.

How to convert rasters to polygons

Raster to Polygon In QGIS:

1. Load the raster layer into QGIS
2. Use the “Polygonize” tool under the “Raster” menu
3. Select the input raster layer and choose the output vector layer
4. Run the tool and the raster will be converted to polygon features

Raster to Polygon In ArcGIS:

1. Load the raster layer into ArcGIS
2. Use the “Raster to Polygon” tool under the “Conversion” toolbox
3. Select the input raster layer and choose the output vector layer
4. Run the tool and the raster will be converted to polygon features

Raster to Polygon In GDAL

To polygonize a raster using GDAL (Geospatial Data Abstraction Library), you can use the following command line:

gdal_polygonize.py input_raster.tif output_vector.shp -f "ESRI Shapefile" 

Where:

• input_raster.tif is the name of your input raster file.
• output_vector.shp is the name of the output vector file.
• -f "ESRI Shapefile" specifies the output file format.

The gdal_polygonize.py the tool will convert the raster into a vector format, with polygon features representing the different classes or values in the raster.

How to convert rasters to Polylines

It’s important to note that the quality of the vectorized lines will depend on the resolution and characteristics of the input raster data. If the input raster data has a low resolution or does not contain clear lines, the output vector data may not be satisfactory.

In such cases, additional pre-processing or post-processing may be required to improve the quality of the vectorized data.

Raster to polylines in QGIS

To convert a raster into polylines in QGIS, you can use the “Raster to Vector” tool, which is available in the Processing Toolbox. The following steps outline the basic process of converting a raster into polylines in QGIS:

1. Load the raster into QGIS.
2. Go to the Processing Toolbox, and select the “Raster to Vector” tool.
3. In the “Raster to Vector” tool, select the raster layer you want to convert and specify the output format as “Lines”.
4. Optionally, you can set a threshold value to control the minimum width of the lines to be extracted.
5. Run the tool and the output will be a new vector layer of polylines.

Raster to polylines in ArcGIS

To convert a raster into polylines in ArcGIS, you can use the “Raster to Polyline” tool, which is available in the Spatial Analyst toolbox. The following steps outline the basic process of converting a raster into polylines in ArcGIS:

1. Load the raster into ArcGIS.
2. Go to the Spatial Analyst toolbox, and select the “Raster to Polyline” tool.
3. In the “Raster to Polyline” tool, select the raster layer you want to convert and specify the output format as “Polyline”.
4. Optionally, you can set a threshold value to control the minimum width of the lines to be extracted.
5. Run the tool and the output will be a new vector layer of polylines.

Raster to polylines in GDAL

This is typically done by applying a contour line or isoband algorithm to the raster, which generates polylines representing equal values or ranges of values in the raster.

In GDAL, you can use the following command to convert a raster into polylines:

gdal_contour -a Elevation input_raster.tif output_polylines.shp

In this example, “input_raster.tif” is the name of the input raster file, “output_polylines.shp” is the name of the output polyline file, and “-a Elevation” specifies the attribute in the raster that the polylines should be generated from (in this case, elevation).

The result of this conversion will be a polyline layer, with each polyline representing a contour line or isoband in the original raster. The attributes of each polyline will typically include the value or class assigned to that line in the raster.

How to convert rasters to points in GIS

This process involves creating a point feature for each cell in the raster, with the attributes being the value or class assigned to that cell in the original raster.

Raster to Points in QGIS

1. Load the raster layer into QGIS
2. Use the “Point Sampling Tool” under the “Processing” menu
3. Select the input raster layer and choose the output point layer
4. Run the tool and the raster will be converted to point features

Raster to points in ArcGIS

1. Load the raster layer into ArcGIS
2. Use the “Raster to Point” tool under the “Conversion” toolbox
3. Select the input raster layer and choose the output point layer
4. Run the tool and the raster will be converted to point features

Raster to points in GDAL

o convert a raster to points using GDAL, you can use the following command:

gdal_translate -of XYZ input_raster.tif output_points.csv

In this example, “input_raster.tif” is the name of the input raster file, and “output_points.csv” is the name of the output point file. The “-of XYZ” option specifies that the output file format should be CSV (Comma Separated Values) with X, Y, and Z values for each point.

The result of this conversion will be a CSV file with one row for each cell in the raster, and three columns representing the X and Y coordinates of the cell center and the value or class assigned to that cell in the original raster.

In conclusion

Vectorization is an important process in GIS that allows us to convert raster data into vector data, making it easier to analyze and visualize geographic information. Whether you’re working with elevation data, land use maps, or RGB imagery, there are various tools and methods available for converting rasters into vectors, such as polygonization, conversion into points, and processing of multiband rasters.

It’s important to note that the specific steps required for vectorization will vary based on the data and use case. However, by understanding the basics of vectorization and familiarizing yourself with the available tools and methods, you can effectively convert rasters into vectors and gain new insights into your geographic data.

In this article, we have explored the basics of vectorization and provided an overview of the methods and tools available for converting rasters into vectors. We hope that this information will be helpful in your work with geospatial data and that it will inspire you to explore the exciting world of GIS and vectorization.