GEOG 480
Exploring Imagery and Elevation Data in GIS Applications

Lesson 3 Introduction


Remote sensing, as a broad discipline within geospatial science, extracts two types of information from images: thematic (what is it?) and positional (where is it?). Thematic information is extracted through a process of image interpretation and analysis; positional information is extracted through the process of creating maps from remotely sensed data. In Lesson 2, we set the stage to discuss maps and mapping by providing a background in datums, coordinate systems, and georeferencing technology. In Lesson 3, we will begin to connect those concepts with the remotely sensed data itself, concentrating on the aerial photograph; however, we will see in later lessons how these principles are applied to elevation data.

Photogrammetry is defined as "the art, science, and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring, and interpreting photographic images and patterns of electromagnetic radiant energy and other phenomena. Photogrammetry provides the positional half of the information equation described above. In Lessons 3 - 5, we will concentrate on the photogrammetric principles of precise and accurate measurement that are essential to the creation of good base maps for GIS. In later lessons, we will introduce foundations of interpretation and image analysis that are also an important application of remote sensing. In this lesson, you will learn more about the special geometric relationships between overlapping aerial photographs, which allow creation of an accurate three-dimensional depiction of the ground. Odd as it may seem, we need this accurate three-dimensional model before a spatially accurate two-dimensional image base map (an orthophoto) can be generated. As was mentioned in Lesson 2, the third dimension, elevation, is needed in order to remove relief displacement from the source imagery.

This lesson will also introduce key elements of photogrammetric project planning, including constraints of lighting, weather, and season that apply to all types of passive optical sensors. You will be introduced to the techniques and methods of data extraction using specialized photogrammetric instruments and software, and you will learn to identify common image-based GIS data products. Finally you will use Internet data sources to find and download various types of aerial photography, and you will create an orthophoto base map using a raw aerial photo and a digital elevation model.

Lesson Objectives

At the end of this lesson, you will be able to:

  • describe the basic photogrammetric concepts used in orthorectification of imagery.
  • explain the difference between simple georeferencing and rigorous orthorectification.
  • perform both simple georeferencing and rigorous orthorectification of both airborne and satellite imagery.
  • use web-based tools to locate and download remotely sensed imagery. 
  • identify common image data formats and perform conversions from one format to another.
  • overlay imagery data with vector data layer to prepare for visualization and analysis.


If you have any questions now or at any point during this week, please feel free to post them to the Lesson 3 Questions and Comments Discussion Forum in Canvas.