Reading GeoTiffs

This tutorial will go over how to read GeoTiff files using GeoTrellis on your local filesystem. It assumes that you already have the environment needed to run these examples. If not, please see our Setup Guide to get GeoTrellis working on your system. Also, this tutorial uses GeoTiffs from the raster project from GeoTrellis. If you have not already done so, please clone GeoTrellis here so that you can access the needed files.

One of the most common methods of storing geospatial information is through GeoTiffs. This is reflected throughout the GeoTrellis library where many of its features can work with GeoTiffs. Which would mean that there would have to be many different ways to read in GeoTiff, and indeed there are! In the following document, we will go over the methods needed to load in a GeoTiff from your local filesystem.

Before we start, open a Scala REPL in the Geotrellis directory.

Reading For the First Time

Reading a local GeoTiff is actually pretty easy. You can see how to do it below.

import geotrellis.raster.io.geotiff.reader.GeoTiffReader
import geotrellis.raster.io.geotiff._

val path: String = "path/to/geotrellis/raster/data/geotiff-test-files/lzw_int32.tif"
val geoTiff: SinglebandGeoTiff = GeoTiffReader.readSingleband(path)

And that’s it! Not too bad at all really, just four lines of code. Even still, though, let’s break this down line-by-line so we can see what exactly is going on.

import geotrellis.raster.io.geotiff.reader.GeoTiffReader

This import statement brings in GeoTiffReader from geotrellis.raster.io.geotiff.reader so that we can use it in the REPL. As the name implys, GeoTiffReader is the object that actually reads the GeoTiff. If you ever wonder about how we analyze and process GeoTiffs, then geotrellis.raster.io.geotiff would be the place to look. Here’s a link.

import geotrellis.raster.io.geotiff._

The next import statement loads in various data types that we need so that we can assign them to our vals.

Okay, so we brought in the object that will give us our GeoTiff, now we just need to supply it what to read. This is where the next line of code comes into play.

val path: String = "path/to/geotrellis/raster/data/geotiff-test-files/lzw_int32.tif"

Our path variable is a String that contains the file path to a GeoTiff in geotrellis.raster. GeoTiffReader will use this value then to read in our GeoTiff. There are more types of paramters GeoTiffReader can accept, however. These are Array[Byte]s and ByteReaders. We will stick with Strings for this lesson, but Array[Byte] is not that much different. It’s just all of the bytes within your file held in an Array.

The last part of our four line coding escapade is:

val geoTiff: SinglebandGeoTiff = GeoTiffReader.readSingleband(path)

This line assigns the variable, geoTiff, to the file that is being read in. Notice the geoTiff‘s type, though. It is SinglebandGeoTiff. Why does geoTiff have this type? It’s because in GeoTrellis, SinglebandGeoTiffs and MutlibandGeoTiffs are two seperate subtypes of GeoTiff. In case you were wondering about the second import statement earlier, this is where is comes into play; as these two types are defined within geotrellis.raster.io.geotiff.

Great! We have a SinglebandGeoTiff. Let’s say that we have a MultibandGeoTiff, though; let’s use the code from above to read it.

import geotrellis.raster.io.geotiff.reader.GeoTiffReader
import geotrellis.raster.io.geotiff._

// now a MultibandGeoTiff!
val path: String = "path/to/raster/data/geotiff-test-files/3bands/3bands-striped-band.tif"
val geoTiff = GeoTiffReader.readSingleband(path)

If we run this code, what do you think will happen? The result may surprise you, we get back a SinglebandGeoTiff! When told to read a SinglebandGeoTiff from a MultibandGeoTiff without a return type, the GeoTiffReader will just read in the first band of the file and return that. Thus, it is important to keep in mind what kind of GeoTiff you are working with, or else you could get back an incorrect result.

To remedy this issue, we just have to change the method call and return type so that GeoTiffReader will read in all of the bands of our GeoTiff.

val geoTiff: MultibandGeoTiff = GeoTiffReader.readMultiband(path)

And that’s it! We now have our MutlibandGeoTiff.

Beginner Tip

A good way to ensure that your codes works properly is to give the return data type for each of your vals and defs. If by chance your return type and is different from what is actually returned, the compiler will throw an error. In addition, this will also make your code easier to read and understand for both you and others as well. Example:

val multiPath = "path/to/a/multiband/geotiff.tif"

// This will give you the wrong result!
val geoTiff = GeoTiffReader.readSingleband(multiPath)

// This will cause your compiler to throw an error
val geoTiff: MultibandGeoTiff = GeoTiffReader.readSingleband(multiPath)

Before we move on to the next section, I’d like to take moment and talk about an alternative way in which you can read in GeoTiffs. Both SinglebandGeoTiffs and MultibandGeoTiffs have their own apply methods, this means that you can give your parameter(s) directly to their companion objects and you’ll get back a new instance of the class.

For SinglebandGeoTiffs:

import geotrellis.raster.io.geotiff.SinglebandGeoTiff

val path: String = "path/to/raster/data/geotiff-test-files/lzw_int32.tif"
val geoTiff: SinglebandGeoTiff = SinglebandGeoTiff(path)

There are two differences found within this code from the previous example. The first is this:

import geotrellis.raster.io.geotiff.SinglebandGeoTiff

As stated earlier, SinglebandGeoTiff and MultibandGeoTiff are found within a different folder of geotrellis.raster.io.geotiff. This is important to keep in mind when importing, as it can cause your code not to compile if you refer to the wrong sub-folder.

The second line that was changed is:

val geoTiff: SinglebandGeoTiff = SinglebandGeoTiff(path)

Here, we see SinglebandGeoTiff‘s apply method being used on path. Which returns the same thing as GeoTiffReader.readSingleband(path), but with less verbosity.

MultibandGeoTiffs are the exact same as their singleband counterparts.

import geotrellis.raster.io.geotiff.MultibandGeoTiff

val path: String = "raster/data/geotiff-test-files/3bands/3bands-striped-band.tif"
val geoTiff: MultibandGeoTiff = MultibandGeoTiff(path)

Our overview of basic GeoTiff reading is now done! But keep reading! For you have greater say over how your GeoTiff will be read than what has been shown. - - -

Expanding Our Vocab

We can read GeoTiffs, now what? Well, there’s actually more that we can do when reading in a file. Sometimes you have a compressed GeoTiff, or other times you might want to read in only a sub-section of GeoTiff and not the whole thing. In either case, GeoTrellis can handle these issues with ease.

Dealing With Compressed GeoTiffs

Compression is a method in which data is stored with fewer bits and can then be uncompressed so that all data becomes available. This applies to GeoTiffs as well. When reading in a GeoTiff, you can state whether or not you want a compressed file to be uncompressed or not.

import geotrellis.raster.io.geotiff.reader.GeoTiffReader
import geotrellis.raster.io.geotiff._

// reading in a compressed GeoTiff and keeping it compressed
val compressedGeoTiff: SinglebandGeoTiff = GeoTiffReader.readSingleband("path/to/compressed/geotiff.tif", false, false)

// reading in a compressed GeoTiff and uncompressing it
val compressedGeoTiff: SinglebandGeoTiff = GeoTiffReader.readSingleband("path/to/compressed/geotiff.tif", true, false)

As you can see from the above code sample, the first Boolean value is what determines whether or not the file should be decompressed or not. What does the other Boolean value for? We’ll get to that soon! For right now, though, we’ll just focus on the first one.

Why would you want to leave a file compressed or have uncompressed when reading it? One of the benefits of using compressed GeoTiffs is that might lead to better performance depending on your system and the size of the file. Another instance where the compression is needed is if your file is over 4GB is size. This is because when a GeoTiff is uncompressed in GeoTrellis, it is stored in an Array. Anything over 4GB is larger than the max array size for Java, so trying read in anything bigger will cause your process to crash.

By default, decompression occurs on all read GeoTiffs. Thus, these two lines of code are the same.

// these will both return the same thing!
GeoTiffReader.readSingleband("path/to/compressed/geotiff.tif")
GeoTiffReader.readSingleband("path/to/compressed/geotiff.tif", true, false)

In addition, both SinglebandGeoTiff and MultibandGeoTiff have a method, compressed, that uncompresses a GeoTiff when it is read in.

SinglebandGeoTiff.compressed("path/to/compressed/geotiff.tif")
MultibandGeoTiff.compressed("path/to/compressed/geotiff.tif")

Streaming GeoTiffs

Remember that mysterious second parameter from earlier? It determines if a GeoTiff should be read in via streaming or not. What is streaming? Streaming is process of not reading in all of the data of a file at once, but rather getting the data as you need it. It’s like a “lazy read”. Why would you want this? The benefit of streaming is that it allows you to work with huge or just parts of files. In turn, this makes it possible to read in sub-sections of GeoTiffs and/or not having to worry about memory usage when working with large files.

Tips For Using This Feature

It is important to go over the strengths and weaknesses of this feature before use. If implemented well, the WindowedGeoTiff Reader can save you a large amount of time. However, it can also lead to further problems if it is not used how it was intended.

It should first be stated that this reader was made to read in sections of a Geotiff. Therefore, reading in either the entire, or close to the whole file will either be comparable or slower than reading in the entire file at once and then cropping it. In addition, crashes may occur depending on the size of the file.

Reading in Small Files

Smaller files are GeoTiffs that are less than or equal to 4GB in isze. The way to best utilize the reader for these kinds of files differs from larger ones.

To gain optimum performance, the principle to follow is: the smaller the area selected, the faster the reading will be. What the exact performance increase will be depends on the bandtype of the file. The general pattern is that the larger the datatype is, quicker it will be at reading. Thus, a Float64 GeoTiff will be loaded at a faster rate than a UByte GeoTiff. There is one caveat to this rule, though. Bit bandtype is the smallest of all the bandtypes, yet it can be read in at speed that is similar to Float32.

For these files, 90% of the file is the cut off for all band and storage types. Anything more may cause performance declines.

Reading in Large Files

Whereas small files could be read in full using the reader, larger files cannot as they will crash whatever process you’re running. The rules for these sorts of files are a bit more complicated than that of their smaller counterparts, but learning them will allow for much greater performance in your analysis.

One similarity that both large and small files share is that they have the same principle: the smaller the area selected, the faster the reading will be. However, while smaller files may experience slowdown if the selected area is too large, these bigger files will crash. Therefore, this principle must be applied more strictly than with the previous file sizes.

In large files, the pattern of performance increase is the reverse of the smaller files. Byte bandtype can not only read faster, but are able to read in larger areas than bigger bandtypes. Indeed, the area which you can select is limited to what the bandtype of the GeoTiff is. Hence, an additional principle applies for these large files: the smaller the bandtype, the larger of an area you can select. The exact size for each bandtype is not known, estimates have been given in the table bellow that should provide some indication as to what size to select.

BandType Area Threshold Range In Cells
Byte [5.76 * 109, 6.76 * 109)
Int16 [3.24 * 109, 2.56 * 109)
Int32 [1.44 * 109, 1.96 * 109)
UInt16 [1.96 * 109, 2.56 * 109)
UInt32 [1.44 * 109, 1.96 * 109)
Float32 [1.44 * 109, 1.96 * 109)
Float64 [3.6 * 108, 6.4 * 108)

How to Use This Feature

Using this feature is straight forward and easy. There are two ways to implement the WindowedReader: Supplying the desired extent with the path to the file, and cropping an already existing file that is read in through a stream.

Using Apply Methods

Supplying an extent with the file’s path and having it being read in windowed can be done in the following ways:

val path: String = "path/to/my/geotiff.tif"
val e: Extent = Extent(0, 1, 2, 3)

// supplying the extent as an Extent

// if the file is singleband
SinglebandGeoTiff(path, e)
// or
GeoTiffReader.readSingleband(path, e)

// if the file is multiband
MultibandGeoTiff(path, e)
// or
GeoTiffReader.readMultiband(path, e)

// supplying the extent as an Option[Extent]

// if the file is singleband
SinglebandGeoTiff(path, Some(e))
// or
GeoTiffReader.readSingleband(path, Some(e))

// if the file is multiband
MultibandGeoTiff(path, Some(e))
// or
GeoTiffReader.readMultiband(path, Some(e))

Using Object Methods

Cropping an already loaded GeoTiff that was read in through Streaming. By using this method, the actual file isn’t loaded into memory, but its data can still be accessed. Here’s how to do the cropping:

val path: String = "path/to/my/geotiff.tif"
val e: Extent = Extent(0, 1, 2, 3)

// doing the reading and cropping in one line

// if the file is singleband
SinglebandGeoTiff.streaming(path).crop(e)
// or
GeoTiffReader.readSingleband(path, false, true).crop(e)

// if the file is multiband
MultibandGeoTiff.streaming(path).crop(e)
// or
GeoTiffReader.readMultiband(path, false, true).crop(e)

// doing the reading and cropping in two lines

// if the file is singleband
val sgt: SinglebandGeoTiff =
  SinglebandGeoTiff.streaming(path)
  // or
  GeoTiffReader.readSingleband(path, false, true)
sgt.crop(e)

// if the file is multiband
val mgt: MultibandGeoTiff =
  MultibandGeoTiff.streaming(path)
  // or
  GeoTiffReader.readMultiband(path, false, true)
mgt.crop(e)

Conclusion

That takes care of reading local GeoTiff files! It should be said, though, that what we went over here does not just apply to reading local files. In fact, reading in GeoTiffs from other sources have similar parameters that you can use to achieve the same goal.