Open Source GIS: A GRASS GIS Approach

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GRASS has been under continuous development since [2] and has involved a large number of federal US agencies, universities, and private companies. The core components of GRASS and the management of integration of efforts into its releases was originally directed by the U. Army Corps of Engineers , in Champaign, Illinois. A key motive was the National Environmental Policy Act. The development platform was Unix running on VAX hardware.

Around this time, a port of the software to Linux was made. Since then, GRASS has evolved into a powerful software suite with a wide range of applications in many different areas of scientific research and engineering. For example, it is used to estimate potential solar photovoltaic yield with r. It was released in , replacing the old stable branch 6. From Wikipedia, the free encyclopedia. This article is about computer software. For plants growing in the ground, see Grass. Free and open-source software portal.

New York: Springer. Bangkok, Thailand. Archived from the original PDF on Retrieved on Solar Energy. The solar radiation model for Open source GIS: implementation and applications. These points, which can be digitized on a reliable map or acquired by GPS on the field, are projected perpendicularly to the channel centreline and reported along the x-axis of the graphics with their distances from one another and their cumulative distances from the origin.

In most works, the channel centreline, i. In a few works, specific algorithms have been proposed to automatically derive this feature by thinning a channel raster map pixel by pixel [4] [5] or to extract it from the bank lines in vector form. Figure 1. Portion of the vector map showing the limits of the bankfull channel and of longitudinal and lateral bars, which is the input map for the calculation of morphometric parameters.

In [7] the centreline has been defined by joining a set of evenly- spaced points along the river channel, whose equidistance from the two banks has been determined by successive approximations. However, the module gives good results when the lines are approximately parallel to each other.

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Moreover, it may give unreliable results when two reaches of the channel are very close to each other, as can happen at the neck of a meander. In this work, a different approach is adopted and the centreline is extracted from the vector map containing the bankfull channel limits, manly by using the GRASS module that traces lines parallel to existing features. Starting from the bankfull channel margins, pairs of lines parallel to the channel banks are repeatedly traced out toward the channel centre. Since each of the two parallels traced out at each iteration is equidistant from the pertaining bank line, the intersection of the two parallels defines points that are equidistant from the banks Figure 2 a1 - a8.

The line joining these intersection points defines the channel centreline Figure 2 b. The length of the centreline is assumed as the length of the channel. Figure 2.

Procedure for channel centreline definition and channel transects tracing. The channel width is commonly defined as the length of the line from bank to bank orthogonal to the channel centreline. In the above mentioned algorithms [4] [5] , channel width is automatically computed along the lines orthogonal to the pixels forming the computed raster centreline.

In [8] , a Python script is described for the calculation of channel width at regularly-spaced transects orthogonal to the channel centreline. The same procedure is adopted in the script described in this work Figure 2 c , Figure 2 d.

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Many different indexes have been proposed by various authors to express the degree of braiding of a river [9]. Among them, the so-called Channel count index [10] is the most commonly used one, since it is simple to calculate and is the least sensitive to river-stage effects [11]. It is simply expressed as the number of active subchannels along a channel transect. Similarly to what is reported in the above cited paper [8] , in the script described herein this index is computed by simply counting the number of active channels along the same transects where the channel width is defined.

In its classical formulation, the sinuosity index is defined as the ratio of channel length to valley length [12].

It requires prior subdivision of the river valley into rectilinear segments. Then, the length of the channel centreline encompassed in the valley segment is divided by the length of the valley segment. The valley segmentation introduces a certain degree of subjectivity, especially where the valley shows a curvilinear path. To reduce the degree of subjectivity, in [8] a fixed length value is used for the valley segmentation. In this work, to further reduce subjectivity and obtain a more detailed assessment of sinuosity along the entire channel course, the river valley is ignored and only the channel centreline is used.

More precisely, sinuosity is calculated by considering a portion of the channel centreline with fixed length, progressively shifted downstream by a constant distance. Sinuosity is obtained by dividing the fixed length of the centreline tract by the straight-line between its endpoints. The computed sinuosity value is assigned to the midpoint of the centreline tract Figure 3. Figure 3. Procedure for sinuosity calculation by dividing the fixed length of a centreline tract by the distance between its endpoints. The centreline tract is progressively shifted downstream by a fixed length along the entire centreline and the computed sinuosity value is assigned to the midpoint of the tract.

Figure 4. Example of screenshot of simple graphs of the computed width a , braiding b , sinuosity c and lateral shifting d. In each file, the first column reports the progressive number of each measure, the second reports the distance from the origin and the third reports the parameter value Table 2. The second script computes the channel centreline lateral shifting between two dates. As it requires the pre- emptive construction of the channel centrelines as of the two dates by the previously described shell script, a specific shell script has been constructed.

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The script, starting from a vector map containing the channel centrelines as of the two dates, computes the lateral shifting using a procedure similar to the previously described one adopted to define the centreline between the two channel banks. Indeed, the parallels to the two centrelines are traced out and their intersection joined by a line that represents the line equidistant from the two centrelines. The distances between the two centrelines are then computed along lines orthogonal to the computed axis at regular intervals and are assumed as the channel centreline shifting between the two dates Figure 5.

The output consists of a report Table 3 and of a text file with the shifting values at regular interval along the computed centreline axis Table 2. A rough simple graph is also displayed on the screen Figure 4 d. For the creation of graphs showing the variations of the parameters along the river channel, two scripts were built. The first one constructs graphs of the four calculated parameters width, braiding, sinuosity and lateral shifting as of a specific date. The input includes the vector map containing the channel centreline, the vector map containing the reference points and the four text files containing the parameters previously computed by the first script.

In addition to the curves of the original computed values, curves of simple moving average of various orders can also be drawn. Obviously, vertical and horizontal scales, size and colours of the lines and characters can be defined by the user. Examples of the curves of the four parameters are given in Figure 6. Table 1. Example of output report containing the main computed statistics.

Table 2. Example of text files with the computed values of channel parameters only the first and last 10 values of each file are reported. Table 3. Example of an output report resulting from the centreline lateral shifting calculation between two dates the example refers to the time interval in [1]. Figure 5. Procedure for lateral shifting calculation.

The axis of the centreline position as of the two dates is calculated and the distance along the lines orthogonal to the axis is assumed as the shifting value. Figure 6.

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Examples of the curves of the four channel parameters: width a , braiding b , sinuosity c refer to the date, shifting d to the time interval [1]. In addition to the curves of the original values, the moving average curves of 21 terms are traced thicker red lines. In the lower x-axis the progressive distances in meters from the origin are reported. On the upper x-axis the positions of the reference points numbered from R01 to R57 projected on the channel centreline are reported, together with their partial and cumulative distances.

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The second script draws the curves of each single parameter as of several dates in a single graphic, allowing an overview of the progressive variations of each parameter over time. An example showing moving average curves of the channel width as of several dates is reported in Figure 7. The input and output of the four shell scripts are schematically summarized in Table 4.

The use of shell scripts based on the GRASS GIS commands allows fast and automatic calculation of the main morphological parameters and the drawing of graphs which detail the continuous parameters variation along the. Figure 7. Example of multiple dates graphic. Curves refer to the channel width as of the nine dates from to considered in [1].

Open Source GIS: A GRASS GIS Approach Open Source GIS: A GRASS GIS Approach
Open Source GIS: A GRASS GIS Approach Open Source GIS: A GRASS GIS Approach
Open Source GIS: A GRASS GIS Approach Open Source GIS: A GRASS GIS Approach
Open Source GIS: A GRASS GIS Approach Open Source GIS: A GRASS GIS Approach
Open Source GIS: A GRASS GIS Approach Open Source GIS: A GRASS GIS Approach
Open Source GIS: A GRASS GIS Approach Open Source GIS: A GRASS GIS Approach

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