Spatial Data Visualization and Modeling
A Digital Elevation Models (DEMs) are collections of elevation points for an area. Certain software can display Digital Elevation Models to show the terrain surface in three dimensions and from a choice of viewpoints. There are currently two methods that are being used to gather elevation coordinates: the systematic grid pattern (regular grid) and stratified method of data collection (irregular grid). A regular grid has equally spaced points regardless of the terrains shape while an irregular grid uses points of larger percentage in areas where there are more relief variation like mountains and valleys. The accuracy of a DEM is greatly affected by the spacing and density of the elevation points. The model’s accuracy depends on the density of the recorded elevation points. Survey or elevation collection can be done using traditional surveying methods or Global Positioning System (GPS). However, in most cases, the elevations are being calculated from contour lines on pre-existing maps. Recently, DEMs are created directly from photographs. These DEMs are later used to create contours. Selecting points is a major issue in building a DEM. Very dense matrix of points may be needed to capture the land surface with variations. The figure below gives a comparison between a 2-Dimentional Topographic Map and a 3-Dimentional DEM. It can be observed that a DEM offers a clearer and more accurate view of the terrain.
2D Topographic Maps VS 3D DEMs
Photo taken from <>
In the last 20 years, DEMs have become a widely used tool and product. They provide elevation values and a snap shot of the landscape and landscape features. Digital Elevation Models have allowed us to better visualize and interrogate topographic features. I chose Digital Elevation Models to produce an inclined contour map because DEMs are more reliable and more widely used. There are a lot of available materials for the project. It is more easy to use DEMs in data visualization and modeling. There is a variety of techniques to create a more effective spatial visualization model using Digital Elevation Models. Existing points are being used by the Interpolation algorithms to estimate the elevations of points that lie between known elevation points. A continuous three-dimensional surface can be achieved by using interpolation algorithms. The interpolation algorithms are used by the 3-D modeling software of the computer to create a more accurate model.
Objective:
The objective of this project is to create a Spatial Visualization Model using DEMs. This project seeks to stress the reliability and effectivity of Digital Elevation Models in creating an inclined contour map.
Background of the Study:
Prior to computers, it was hard to display and analyze elevation models because they can be just compiled as lists or tables. The computer software plays an important role in DEM analysis. The software must not only display the model, but it must allow for slope calculation, volumes, and other three 3D qualities in order for the data to be fully useful. Now, there are software that can enhance the quality of the Digital elevation Models. A spatial analysis can be made by integrating DEMs and High-end GIS software. 3-D modeling enhances projects such as land erosion susceptibility, analysis of view sheds, prediction of floods, mining potential, site selection and others. The figures below display one of the useful properties of DEM. Digital Elevation Models can reclassify data sets to meet the requirements or parameters of landscape components.
Aspect Classification Slope Classification
3-D Digital Elevation Model.
Photos taken from <www.dpi.vic.gov.au>
Digital Elevation Models can be used to display three-dimensional graphic images like terrain slope and terrain profiles between selected points. DEMs can be combined with digital raster graphics, digital line graphs and digital orthophoto quadrangles to enhance the visual information and to create pleasing and dramatic hybrid digital images.
Methodology:
There are several methods to create DEM. One method is the conversion of printed contour lines in which we scan existing plates used for printing maps. The raster is then vectorized and edited. Then these contours are tagged with elevations. Additional elevation data are created from the hydrography layer. Finally, an algorithm is used at every grid point from the contour data to interpolate elevations. Another method is using photogrammetry that can be done manually or automatically.
The elevation contours on the topographic map must be converted to xyz data in order to create a DEM from a topographic map. The raster elevation contours must be first converted to vectors. The corresponding elevation values must be tagged to the elevation values. Using an interpolation algorithm, the tagged data is transferred to a superimposed grid. The gridded elevation values are then, converted into GIS format for it to be used by other applications.
A topographic map with a suitable color raster image must be obtained. These high-resolution raster topo is sometimes available in TIF or other graphic file format or sometimes it can be downloaded from the internet. The layers must be separated. It is necessary to use image-processing techniques to achieve this. The image must be filtered to separate the elevation contours from unwanted raster data. Now, the vectors must be inspected for correctness. When the repairs are completed, the contours must be tagged. Once the contours are tagged, the next step is to convert it to a DEM file. The final image is shown below.
Photo taken from <>
Three-Dimensional images can be rendered from Digital Elevation Models requires a certain level of skill and art. POV-Ray is a general-purpose 3-D rendering tool. POV-Ray can be used if you convert a DEM file in ASCII or SDTS format to TGA file. POV-RAY interprets the TGA file to produce a 3D image. After the conversion, the resulting TGA will be imported into the ray tracer POV-RAY.
3DEM is a Digital Elevation Model program that can be downloaded free. This program can overlay USGS topo maps over DEMs. The first step is to log on to and download a SDTS DEM of your interest (only in USA). Sample image is shown below
Photo taken from <>
Factors affecting the quality of Dem-derived products:
The accuracy of elevation and the accuracy of the morphology are used to measure the quality of a Digital Elevation Model. Several factors affect the quality of a DEM-derived product. These are the following: Terrain Roughness, density of the sample, grid resolution, interpolation algorithm, vertical resolution and terrain analysis algorithm.
Uses of Digital Elevation Models:
DEMs can be used to determine the attributes of terrain such as elevation, slope and aspect. DEMs are also useful in searching features on the terrain such as watersheds, drainage basins, peaks and pits or other landforms. DEMs are also useful in modeling forest fires, energy flux and hydrologic functions. Digital Elevation models can also be used in 3D visualizations, satellite imagery and in creating relief maps. DEMs are also useful in extracting terrain parameters, creation of physical models and can be used to analyze terrains.
Soft photogrammetry is one of the technologies that were developed to satisfy the demands for high accuracy elevation and terrain information. Tonal imbalances are removed by using color matching and radiometric leveling.
Conclusion:
Making an inclined contour map requires expertise and skills. Digital Elevation Models are important in making an inclined contour map. Digital Elevation Models are reliable and most of the time more accurate than 2D topographic maps. There are different software, programs and techniques in making a DEM. The accuracy of a DEM depends on the density of the recorded elevation points. Today, DEMs are widely used by governments all over the world.
Sources:
Credit:ivythesis.typepad.com
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