====== Fast flow accumulation ======

This project will improve the speed of flow accumulation.

===== Preliminary results =====

  * Systems
    * Linux 4.14.39 with SSD and 16GB RAM on Yoga 370
    * ArcGIS on UNG Windows with SSD and 32GB RAM
  * Data: Texas drain raster from [[efficient_longest_flow_path_algorithm|Efficient longest flow path algorithm]]
  * Benchmark algorithms from https://github.com/zhouguiyun-uestc/FastFlow

^ Program                                                      ^ Algorithm only (sec)  ^ Total (sec)  ^
| ''FastFlow Zhou fdr.tif fac.tif''                            |  61                   |  87          |
| ''FastFlow Wang fdr.tif fac.tif''                            |  174                  |  277         |
| ''FastFlow Recursive fdr.tif fac.tif''                       |  178                  |  204         |
| ''FastFlow Jiang fdr.tif fac.tif''                           |  304                  |  333         |
| ''FastFlow BTI fdr.tif fac.tif''                             |  118                  |  144         |
| TauDEM ''aread8 -p taud8.tif -ad8 tauad8.tif''               |  402                  |  607         |
| TauDEM ''mpiexec -n 2 aread8 -p taud8.tif -ad8 tauad8.tif''  |  310                  |  514         |
| ArcGIS Flow Accumulation                                     |                       |  478         |
| Arc Hydro Flow Accumulation                                  |                       |  681         |
| **Coming soon**                                              |  **48**               |  **107**     |
| **Yet another coming soon**                                  |  **21**               |  **33**      |

===== Results =====

{{https://hcho.isnew.info/research/fig-fdr.png?400}}
{{https://hcho.isnew.info/research/fig-benchmark-results-compute.png?400}}

===== Discussion =====

Too much overhead for raster I/O. Need to reimplement I/O using [[https://gdal.org/tutorials/raster_api_tut.html|GDAL]] or [[https://docs.fileformat.com/image/tiff/|from scratch]]? Done using GDAL.

===== References =====

  * [[http://skl.iswc.cas.cn/zhxw/xslw/201811/P020181116568187439544.pdf|A fast and simple algorithm for calculating flow accumulation matrices from raster digital elevation models]] (https://doi.org/10.1007/s11707-018-0725-9)
  * [[https://gdal.org/tutorials/raster_api_tut.html|Raster API tutorial]]
  * [[https://docs.fileformat.com/image/tiff/|TIFF - Image File Format]]
  * [[https://medium.com/planet-stories/reading-a-single-tiff-pixel-without-any-tiff-tools-fcbd43d8bd24|Reading a Single TIFF Pixel Without Any TIFF Tools]]

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