Problems. For some years it has been possible with digital photogrammetric workstations (DPWs) to automatically acquire digital elevation models (DEMs) from medium-scale aerial photography (1:20,000-1:40,000); recently, however, there has been a tremendous increase in the use of large-scale imagery (1:5,000-1:10,000). For such scales, automatic DEM acquisition fails. The present DPW software are not open to build new models. At large scales, automatic stereo correlation fails when the terrain contains man-made structures (i.e., buildings, bridges, etc.). Large-scale imagery exhibits severe distortions along linear features. All of these problems are linked to a misconception of DEMs, represented as regular array. We need triangular irregular networks (TINs) in order to include breaklines. We need algorithms to extract linear features and algorithms to direct where correlation has to be done. In fact, we are looking for an image analysis development tool which is, strangely enough, absent in all DPWs. A macro language is lacking. The main paradox of the present DPW lies in the concept of photogrammetry itself. The goal of photogrammetry is to gather data in any GIS. The present situation is that no direct links exist between GIS and DPW.
Solutions. It is shown that the grid module can be used to display stereo images and stereo-orthos. An anaglyph solution is obtained using the stack concept. It is shown that the grid module can be used to define new interest points, to extract breaklines, and to perform image segmentation. It is shown that a macro language (AML) and the grid language are the tools needed to model problems in digital photogrammetry. It is shown that ArcInfo offers an ideal platform to raster/vector interactions. It is shown that digital photogrammetry and GIS have a common language based on spatial relationships. The ArcTools environment plays a central role in managing a DPW. The view tools (under grid or ARCPLOT) allow vector superposition of different coverages (point, line, etc.). We switch quickly from mono to stereoscopic or 3-D representations. Such tool is essential for a quality check process. Finally, an overview of new functionalities to be built will be listed.
Most spatial data-that is, the lines, points, and polygons central to GIS analysis-are derived from aerial photographs or satellite images. But we are often unaware of the photogrammetry used for capturing the feature and terrain information. Instead, we use paper or digital maps that already contain the necessary feature information. But with advances in technology, it is becoming feasible for the GIS user to obtain the feature and terrain information directly from the source-that is, the aerial photographs and satellite images. Photogrammetry uses overlapping images to mathematically convert the coordinates of features within an image into real-world coordinates such as latitude or longitude (known as ground space). Digital stereo workstations automate much of the process for registering images to ground space, creating orthophotos, extracting features, and creating digital elevation models (DEMs). Allowing a GIS direct access to photogrammetric software provides many benefits. Accuracy is greatly improved, both by eliminating steps in the data acquisition process and by reducing the amount of generalization and data transformations. The cost and time required for acquiring good data is reduced. DEMs provide elevation data as well as visual aids such as perspective scenes. By focusing on the interaction between photogrammetry and GIS, we can gain a deeper understanding for the requirements and value of data needed for our spatial analysis. Furthermore, constant interaction between the two systems will provide a GIS user with more control over the entire process for handling, analyzing, and presenting spatial data. So the question still remains-Are paper maps good enough? By the end of this presentation, you should be able to make your own decision.
This presentation will provide a general education about the issues involving digital orthophotography. I will gear my talk toward those who may be interested in acquiring orthophotos as a basemap for their GIS or simply to enhance their next project. I will discuss how to create specifications for digital orthophoto acquisition, covering issue such as accuracy, image resolution, and costs. Other issues that will be covered in this presentation include file size issues, tiling schemes, edge-matching and mosaicking, and image viewing software. I will also discuss the "forgotten" issues that are often left out of Requests for Proposals, but become extremely momentous during deliveries. The audience should come away from this talk with a general understanding of what to ask for and what to expect when ordering digital orthophoto products.