# Concepts¶

## Gridded Data¶

RichDEM assumes that data is provided in the form of a rectangular grid of cells with some width and height. Furthermore, the data comprising this grid must be laid out in a flat array such that the value of any cell (x,y) can be accessed via the equation y*width+x. This is known as row-major ordering.

Data can be passed to RichDEM in one of three ways.

1. Data can be loaded via GDAL. GDAL handles the heavy lifting of ensuring that data is loaded in a form which complies with the above assumptions.
2. Data can be manually added to a richdem::Array2D object (C++) or a richdem.rdarray object (Python).
3. A richdem::Array2D (C++) or richdem.rdarray (Python) object can be used to wrap existing row-major memory. This capability allows RichDEM to easily integrate with existing code.

A RichDEM array is accompanied by several kinds of metadata. These can be loaded by GDAL or specified manually.

• A NoData value, as discussed below.
• A projection. This is, typically, a PROJ4 or WKT string that identifies the projection the data maps to. The choice of projection does not affect RichDEM’s operations.
• A geotransform. This is a six element array which determines where in a projection a RichDEM array’s data is located, as well as the cell sizes. Further details are below. This setting does affect how RichDEM processes data.
• A metadata entry which contains arbitrary metadata strings such as PROCESSING_HISTORY (see below).

## Geotransform¶

A geotransform is a six element array which determines where in a projection a RichDEM array’s data is located, as well as the cell sizes.

Typically, the geotransform is an affine transform consisting of six coefficients which map pixel/line coordinates into a georeferenced space using the following relationship:

Xgeo = GT(0) + Xpixel*GT(1) + Yline*GT(2) Ygeo = GT(3) + Xpixel*GT(4) + Yline*GT(5)

In case of north up images, the GT(2) and GT(4) coefficients are zero, and the GT(1) is pixel width, and GT(5) is pixel height. The (GT(0),GT(3)) position is the top left corner of the top left pixel of the raster.

Note that the pixel/line coordinates in the above are from (0.0,0.0) at the top left corner of the top left pixel to (width_in_pixels,height_in_pixels) at the bottom right corner of the bottom right pixel. The pixel/line location of the center of the top left pixel would therefore be (0.5,0.5)

(Text drawn from GDAL documentation.)

## NoData values¶

RichDEM recognizes cells with a NoData value and treats them in special ways. The NoData value is a number, such as -9999 which represents a cell that isn’t part of a data set.

In depression-filling and the determination of flow directions, NoData cells are treated as having a lower elevation than any other cell, which enforces drainage to the edge of the DEM.

Because NoData values fundamental affect operations, RichDEM requires that you specify what NoData value it should use.

It is important that you choose a value that doesn’t correspond to any of your actual data values. In 1-byte/8-bit DEMs this may not be possible since there are only 256 distinct values available. In this case, you should cast your data to a 2-byte/16-bit form and choose a NoData value that is not in the range 0-255.

Other terrain analysis programs may use a binary masking array to indicate which cells are NoData. RichDEM does not do this because usually a minority of cells are NoData and a separate array increases the amount of memory used and reduces cache utilization, both of which may reduce performance.

Language Set Get
Python rda.no_data=-1 rda.no_data
C++ rda.setNoData(-1) rda.noData( )

## Processing History¶

RichDEM automagically keeps track of what operations are performed on your data. This means that your outputs will contain an exact record of what operations were used to produce them. This helps ensure reproducibility. And, remember, good science is reproducible science.

In the following, a series of operations is performed and the Processing History is then examined.

import richdem as rd
rd.FillDepressions(dem, epsilon=False, in_place=True)
accum = rd.FlowAccumulation(dem, method='D8')
rd.SaveGDAL('/z/out.tif', accum)


The processing history of a saved dataset can be viewed using a few different commands:

gdalinfo /z/out.tif
rd_info  /z/out.tif


The processing history appears as follows.

2017-12-20 17:55:19.892388 UTC | RichDEM (Python 0.0.4) (hash=e02d5e2, hashdate=2017-12-19 23:52:52 -0600) | LoadGDAL(filename=../data/beauford.tif, no_data=-9999.0)
2017-12-20 17:55:19.900234 UTC | RichDEM (Python 0.0.4) (hash=e02d5e2, hashdate=2017-12-19 23:52:52 -0600) | FillDepressions(dem, epsilon=False)
2017-12-20 17:55:20.514098 UTC | RichDEM (Python 0.0.4) (hash=e02d5e2, hashdate=2017-12-19 23:52:52 -0600) | FlowAccumulation(dem, method=D8)


Note that the first column is the time at which the operation was performed, the second column is the program which performed the operation, and the third column is the command which was run.

Language Command
Python rda.metadata
C++ rda.metadata

## In-Place Operations¶

To save memory RichDEM performs some operations, such as depression-filling, in place. This means that the data is modified and the original data will be lost unless it has been copied.

For instance, in Python FillDepressions has two distinct forms:

#In-place filling, no return value
rd.FillDepressions(dem, in_place=True)
#Fill a copy
dem_filled = rd.FillDepressions(dem, in_place=False)


whereas in C++, a copy must be made:

#In-place filling, no return value
richdem::FillDepressions(dem)

#Fill a copy
auto demcopy = dem; //TODO: Make sure this syntax is right
richdem::FillDepressions(demcopy)