Photogrammetry

Laboratory Exercise 7

Goals and Objectives

            This lab exercise opens students to a variety of photogrammetric techniques, and introduces the mathematics behind relief displacement, and the measurement of features within the image. These skills can be harnessed to perform more advanced techniques like the stereoscopic technique of orthorectification of images. These types of skills and background knowledge are becoming increasingly more desirable in a professional setting, so understanding the basics behind the mathematics of relief displacement create a solid foundation on which to build more advanced skills required in today’s job market.

Methods

The beginning section of the laboratory exercise asks students to calculate the amount of relief displacement required for an image of the city of Eau Claire. The relief displacement is calculated in relation to the principle point of the image. Depending on whether the object is positioned above (in elevation), or below the principle point, the object will be displaced in an upwards or downwards fashion. Combing knowledge learned in lecture, along a few mathematical formulas, students manually calculate how much the specific object needed to be rectified.

The next portion of the lab exercise introduces students to the stereoscopic technique of making an anaglyph, and how to appropriately use elevation models to make a pseudo-3D image. Students produced two different anaglyphs of the same image and compared the output for effectiveness. The first anaglyph was created using a DEM (Digital Surface Model), and an anaglyph function within ERDAS imaging. The output of this function was barely noticeable 3D effect to the human eye, but did add some distinction to the elevation change of the image. The second anaglyph made, used a 2m LiDAR DSM (digital surface model). This output was considerable better than the previous anaglyph, and added a noticeable 3D effect when visualizing the elevation using 3d glasses. The 2m resolution provided much more information for the function to work more effectively, but added the computation time of the function. The DSM anaglyph output product is an effective way to visualized and gain a better aspect of the elevation in the image, and where the most elevation change is located.

The Last portion of this lab deals with orthorectifing an image with a reference image. The process of orthorectifing produces an image which has the image perspective effects removed. Processing images by orthorectification removes the tilt and terrain relief to produce a planimetrically corrected image. The process of orthorectification is quite extensive, and involves many different types of connecting points between the image being rectified and the reference image. Essentially, an image photogrammetry program is used in ERDAS to collect ground control points (GCP’s) , both control and tie points on the images. Most importantly, is the collection of points between the two images being rectified. Connecting the images with multiples of points, allows the computer software to produce an output with very limited perspectival anomalies, which produces a seamless and functionally usable image.

Conclusions

Remote sensing is an ever evolving, complex subject, which requires a multitude of skills. Gaining knowledge into the specifics of important techniques like creating an anaglyph, and orthorectified images further separates successful remote sensing professionals from unsuccessful. This lab provided the background information, for students to apply the material learned in lecture, and applied it to very real, and important techniques in the production of usable and applicable images.