In this assignment we will focus on Arc View’s Spatial Analysis extension that including main topic is Raster Modeling and Analysis, which in the real world, especially, about natural resources water directly affects all people. Displaying information about water availability as well as water quality in away that is simple, clear, and understand able to the user is important. The application of GIS technology has made this easier. The hydrology processes that take place in a watershed can be viewed as the movement of water between the atmosphere, the vegetation, and the surface of the land the soil and water bodies and so on.
When rain falls on an area of land, it eventually runs downhill until is reaches a stream. The area over which water flows to a common point is called a watershed. This common point is called outlet of the watershed. Other common terms for a watershed is basin, catchments and contributing area. Traditionally, watersheds have been defined using different map scales and criteria may produce substantially different boundary delineations. With widespread use of GIS watershed boundaries are useful in digital from and at a wide range of map scales so that in this assignment we will be perform in capability of Arc View to determine the contributing area that defines a watershed. We will be created watersheds using a DEM. In the next step we’ll do flow direction, flow Accumulation, Identify Stream Cells, and Label the Links, Determine Watersheds and Burning Stream Data into a DEM. To analyze the watersheds we use analyzing elevation characteristic of two watersheds, analysis with histogram by zone and create summarize zone, calculate mean elevation and calculate area that we will be used the Map Calculator and Avenue scripts. For final task to examine we calculated least cost path analysis comprise polygon –grid conversion, reclassification, assigning weight and creation of cost surface and cost path analysis.
A DEM free sinks, a depression DEM, is the desired input if we wish to created the surface that model flow for every cell in a grid. A DEM is the basis for deriving hydrological products; therefore, it should be as accurate as possible. In some case, there may be legimate sinks in the data. It is important to understand the morphology of the area to know what features may truly be sinks on the surface of the Earth and which are data errors. In this assignment, we will use the Avenue script to iteratively fill the sinks in the input DEM. This script uses the SINK, WATERSHED, FLOWACCUMULATION, and FLOW DIRECTION requests to fill DEMs.
Flow Direction is a direction where the water flows. One of the keys to deriving hydrologic characteristics about a surface is the ability to determine the direction of flow from every cell in the grid. This is done with the FLOWDIRECTION request. This function takes a surface as input and output a grid showing the direction of flow out of each cell. There are eight valid output directions, relating to the eight adjacent cells into which flow could travel.
Maximum drop = change in z value / distance
The distance is determined between cell centers. Therefore, if the cell size is 1, the distance between orthogonal cell is 1 and the distance between two orthogonal cells is 1 and the distance between diagonal cells is 1.4142 (square root of 2). If the descent to all adjacent cells is the same, the neighborhood is enlarged until the steepest decent is found.
When a direction of steepest descent is found, the output cell is coded with the value representing that direction.
Request for Flow Direction: Agrid. FlowDirection (forceEdge)
If forceEdge is TRUE, then all cells on the edge of the surface will flow away from the surface. If forceEdge is FALSE, then flow is calculated normally for edge cells with the edge being slightly lower than the cell.
To discover how much flow is accumulated (gathered) by each cell. The FLOW ACCUMULATION request calculates accumulated flow as the accumulated weight of all cells flowing into each downslope cell in the output grid. If no weight grid is provided, a weight of one is applied to each cell, and the value of cells in the output grid will be to number of cells that flow into each cell.
The aGrid Flow Accumulation request [aGrid. Flow Accumulation (weightGrid)] creates a Grid of accumulated flow to each cell, by accumulating the weight for al cells that flow into each downslope cell.
