Rasterization and Vectorization: Vertices, Paths and Grids

rasterization vectorization

Last Updated: Feb 2, 2017 @ 4:50 am

How is Rasterize Different From Vectorize?

Rasterization refers to converting vectors into rasters. While vectorization transforms rasters in vectors.

We know the two main data models and graphics are vector and raster data.

But how do you go from one data model to the other?

That is when you would use rasterization and vectorization. Learn how to do this for free with open source QGIS. But first for some basics:

READ MORE: GIS Spatial Data Types: Vector vs Raster

What is a Vector?

Vector data are comprised of vertices and paths.

Vector points are simple XY coordinates in space.
Point Vector Data Type
Vector lines connect XY coordinates called vertices with paths.
Vector Data Type Line
Lastly, vector polygons are a set of closed vertices and paths.
Vector Data Type Polygon

What is a Raster?

Unlike vector data, rasters are made up of pixels or grid cells. Further to that, they are usually regularly spaced and square (but they don’t have to be) and often look pixellated.

Discrete rasters are categorical and have distinct values identifying each cell. For example a land cover raster might represent urban as the value 1 and forest as 2.
Discrete raster
Continuous rasters are grid cells with gradual changing data such as elevation, temperature or an aerial photograph. Continuous data is also known as non-discrete or surface data.
Continuous raster

Rasterization: Convert Vector to Raster




In this example, we are going to use the open source program QGIS.

Unlike graphic editing software, it expects your rasters and vectors to have a geographic coordinate system. But really it has similar functionality to Adobe Photoshop or GIMP that you can perform for rasterization and vectorization.

Let’s start with a simple vector line with a vertex at the start and end points.

vector line

Use the  Rasterize (Vector to Raster) Tool . The horizontal and vertical parameters will determine the output pixel size. After setting all the parameters, click RUN for rasterization.

rasterized line

Finally, the output will look like a gridded version of your vector. Depending on the pixel size you set, you may have to do some trial-and-error for better results.

Vectorization: Convert Raster to Vector

Let’s go in reverse this time and take a discrete raster and convert it to vector.

When we have a raster data set like our buildings layer, it’s a matter of adding it to QGIS. Here’s what the raster buildings look like in the table of contents:

raster buildings

QGIS comes loaded with algorithms and plugins for you. Further to this, other providers like GRASS, SAGA GIS and Orfeo toolbox brings additional firepower to the table. In this case, the  vectorization tool (v.to.vect)  in GRASS is the one you’re looking for.

Double-click the tool and select the raster that you want to convert to vector. From here, you’ll have to select the output of the raster to a point, line or polygon. After clicking RUN, the vectorized raster will appear in the table of contents.

vectorized buildings

As a reminder, vectorization creates all the necessary vertices and paths as shown with the building outlines. In a nutshell, this tool digitizes the raster for you converting it to a vector.




If you want to learn how to vectorize a continuous raster, this will take a bit of effort to reclassify cells essentially grouping them into discrete form. Using a temperature raster as an example, when you group values from [-37.8 to 0] as 0 and [0 to 37.7] as 1, you are turning it into a discrete raster. From here, you can follow these exact same steps for vectorization.

READ MORE: How To Vectorize Image Files Automatically with ArcScan

Conclusion

For the most part, it’s a basic process to vectorize or rasterize images.

Rasterization refers to converting vectors into rasters. While vectorization transforms rasters in vectors.

These are the essential steps how to go from one data model to another.

Any questions? Let us know below.

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