ArcView GIS Techniques: DEM, Georeferencing, and Terrain Analysis

Digital Elevation Model (DEM) Creation and GIS Procedures

Understanding the Digital Elevation Model (DEM)

The Digital Elevation Model (DEM) corresponds to a three-dimensional numerical description of the ground surface using derived coordinates. It is typically based on digital contour coverage.

Creating the DEM from Contour Data

The process involves using the 3D Analyst extension:

1. Select Surface Analysis / Create TIN from Feature (Triangular Irregular Network) to generate the elevation model.
2. Convert the model to raster format: Go to Theme / Convert to GRID.
3. Enable the Spatial Analyst extension.
4. Use the new option Analysis / Reclassify to assign the desired elevation ranges.
5. Convert the model back to vector format: Go to Theme / Convert to Shapefile.

Georeferencing Procedures in ArcView

This procedure aligns raster images (like orthophotos) to real-world coordinates using the Image Analysis extension.

1. Enable the extensions Image Analysis and ER Mapper ECW v2.0 and Images.
2. Open the raster file (e.g., .ecw) using Image Data Source.
3. Open the georeferenced orthophoto (e.g., .jpg) using Image Analysis Data Source.
4. Press the Align Tool button to match checkpoints between both images.
5. To place a checkpoint, press "AT" (Align Tool), insert the first point on the photo, and then press the georeferenced image. The tool reverses the order of the pictures, allowing you to place the corresponding point on the georeferenced photo.
6. Save the georeferenced image: Go to Theme / Save Image As.

Digital Editing and Map Layout Design

The map layout procedure involves designing and outputting a final map product (often referred to as a map sheet or 'farm letter').

In a layout, you can add text, tables, and diagrams. Layouts may also be republished, exported, and printed from other programs.

Steps for Creating a Map Layout

1. Choose all necessary layers of information to be used.
2. Determine the required scale.
3. Export the layout to a page definition where the page size and orientation are defined.
4. Within the layout, set and/or add the graphic scale, ensuring it corresponds to the working units.
5. Correct the data ranges in the legend and distribute information clearly.
6. Include a North Arrow (wind rose) depending on the map orientation needs.
7. Grant a title to the site map.

Farm Photo Interpretation for Current Land Use Definition

This process defines current land use based on photo interpretation and GIS digitization.

Preparing the Layout for Interpretation

Once the vector image is created, switch to View / Layout (ensuring the scale is set, preferably in meters). The layout can be formatted using the Graticules and Measured Grids feature.

Land Use Mapping Procedure (Including GPS Data)

This procedure explains how to incorporate current land use coverage and digital terrain data captured with GPS:

1. Create a database with farm information gathering (drawing, picture, etc.).
2. View the scale of the photo; calculate it if necessary.
3. Perform preliminary photo interpretation to complete the Current Use Forms (TOC), which determines use type, species, etc.
4. If the photo is not georeferenced, georeference it.
5. Digitization of the polygons in GIS software (e.g., ArcView).
6. Create the new attribute database.
7. Calculate the surface area, land use percentages, and farm size.

Note: Scale is important; a scale of 1:10,000 is recommended according to CONAF standards.

ArcView Procedures for Terrain Analysis

Mapping Slope from a DEM

1. Open the contours and select the target derived surface: Slope. This generates data in ranges of grades (degrees).
2. Create the data in GRID format.
3. Go to Analysis and select Reclassify, determining the data values to display in the legend.
4. Convert the result to vector format: Go to Theme / Convert To Shapefile to obtain the final slope map.

Mapping Aspect (Exposure)

To obtain the exposure map, perform the same steps as above, but select the option Derive Surface Aspect instead of Slope.

This provides exposure data (East, West, North, and South), with values ranging between 0° and 360°.