Importing large raster datasets
Importing raster datasets in the range of 10 GB or less can usually be done with the default Map Import settings. The general documentation on importing maps can be found here.
However, if the dataset you are importing is significantly larger - 20 GB or more - the settings become more important to get a good result without using too much processing time and file size. This article will go more in-depth on the different parameters and how they affect the import process and result.
Image Quality
Image quality in this context refers to how similar the output result is to the input data. This can be objectively measured as Structural Similarity (SSIM). We have run a benchmark comparing different compression methods and formats here.
Relevant Map Import parameters:
Resampling
This is the method to use when creating downsampled versions of the main dataset (also called pyramids or overviews). The default resampling method is average. The following table shows a benchmark comparing the different methods. Each row shows the percentage difference in execution time from average
| Method | Average time (seconds) | Difference vs. baseline | Sample tile |
|---|---|---|---|
| average | 42.45 | 0% | 
|
| nearest | 38.50 | -9% | 
|
| bilinear | 57.66 | 36% | 
|
| cubic | 61.80 | 46% | 
|
| cubicspline | 60.64 | 43% | 
|
| lanczos | 66.82 | 57% | 
|
| gauss | 54.28 | 28% | 
|
Full article here: Comparing resampling methods in massivegeopackage
Quality
The quality parameter controls how aggresively JPEG tiles are compressed. It is expressed as a value between 10 and 100 where
- Higher values = less compression, larger files, better image quality.
- Lower values = more compression, smaller files, more visible artifacts.
The relationship between file size and visual quality is highly nonlinear. Using quality=100 is not a lossless conversion, but it will result in a very large dataset. Decreasing the number will retain most of the visible quality while giving substantial size reductions. The default value of 75 gives a good balance between visual quality and file size. For a detailed look at the relationship between JPEG quality versus file size, see the article Comparing compression in different formats. The key conclusion from that article is the following plot showing file size compared to structural similarity (visaul quality). Notice how JPEG flattens out at around 60-75 quality.
