Vendor:
ERDAS, Inc.
2801 Buford Hwy., Suite 300
Atlanta, GA 30329-2137
404-248-9000
Fax: 404-248-9400
www.erdas.com

Price Information: 
$3000.00

Distribution Medium: CD-ROM

Hardware:
Pentium-based computer with at least 64 MB of RAM (minimum)

Pentium II-based computer or newer with 128 MB or more of RAM (recommended)

Requires OpenGL 1.1 or higher graphics support (a list of tested and supported cards is available from the vendor)

Monitor capable of refresh rate at 100-120 Hz

Software:
Windows NT 4.0 (sp6) or Windows 2000

Tested Configuration
450 MHz Pentium II with 256 MB of RAM
ELSA GLoria XXL graphics card (supplied by vendor)
NuVision 60GX shuttering glasses with infrared emitter (supplied by vendor)
Windows NT 4.0 (sp6)
 
 

This review is based on using the example imagery and tutorial workbook that come with the software, a digitized stereo pair of large-scale photography, digital high-resolution stereo imagery collected by Emerge, and a quick experiment with terrestrial photographs taken with a digital camera. The tutorial exercises are suitable for users without a photogrammetric background. The training is very detailed, easy to follow, and is supplemented by an extensive introduction to basic photogrammetric concepts. The program is user-friendly and has a short learning curve, but it will still take a while for a novice to become proficient in interpreting and measuring in stereo. (The same is certainly true for the more traditional stereo interpretation of hardcopy imagery.) It took more effort and time to become productive with the other imagery sets, but the reviewers were still able to negotiate the process and extract useful information. Stereo Analyst handled the terrestrial photographs easily, once the photos were acquired with suitable geometry (base/depth ratio, etc.).

Features
Viewing Non-Oriented Digital Stereo Models. At its most basic, the program can be used to view stereo pairs. This works quite well. The photos are aligned on screen and the x and y parallax can be adjusted for proper viewing. However, with this type of non-oriented stereo model, there is limited ability to make measurements and extract information from the imagery. Therefore, most users will wish to use the more powerful features of Stereo Analyst and orient the stereo models so that there is a connection to some type of real world coordinate system. These more powerful features are summarized below.

There are three options for viewing in stereo: Color Anaglyph Stereo Mode, Quad Buffered Stereo Mode, and Line Interleaved Stereo Mode. Color Anaglyph Stereo Mode uses the familiar approach of displaying one stereo image in red and the other stereo image in cyan (blue) and then viewing the computer display with colored glasses. An anaglyph image of the stereo pair can be stored as a single graphics file (easily converted to common formats such as TIFF) for stereo viewing in any graphics program. This feature can be useful in preparing material for a lecture, demonstration, or for use in field interpretation. The second two stereo viewing modes require extra electronic hardware. The Quad Buffered Stereo Mode uses an infrared emitter and special shuttering glasses to rapidly alternate elected right images to the left and right eyes. The Line Interleaved Stereo Mode is designed to be used with polarized display units. The Line Interleaved Stereo Mode was not tested for this review.

Setting Up Oriented Stereo Models. The real power and value of Stereo Analyst comes when using oriented stereo models. The analytical computations needed to solve for the orientation of stereo models must be performed by separate photogrammetric programs. Stereo Analyst provides two methods for then using the orientation information. Stereo Analyst uses ERDAS IMAGINE OrthoBASE’s block file format (.blk) for storing this information. Therefore, importing the block files from IMAGINE OrthoBASE is the easiest way to view stereo models since nothing else needs to be done. The orientation parameters can also be entered manually in Stereo Analyst, but this process demands careful attention. First, the units and coordinate system, including sign convention, must agree with the formats used by Stereo Analyst. There may be some uncertainty with data provided by others. Stereo Analyst accommodates manual entry of orientation information by providing the ability to define units for the image and reference coordinate systems as well as several options for defining rotation angles. Second, not all the data required by Stereo Analyst may be directly available. In addition to the six exterior orientation parameters (X, Y, Z of the exposure station; Omega, Phi, Kappa angles of rotation) and the camera/sensor parameters (focal length, principal point offset), the six coefficients for the affine transformation from pixel/file to image coordinate system are required. While all triangulation programs compute these coefficients, they are an intermediate product and might not be included in the standard output. These also need to be newly computed if a user scans in photographs with previously known exterior orientation. Finally, block files created by directly entering parameters into Stereo Analyst may benefit from the extra step of entering a tie point). This minor task is beneficial when there exists a lot of scale variation within the stereo model.

The end result is that a user might have to do some preparatory work in other programs to get the information necessary for using an oriented stereo model. It’s easiest to do this with IMAGINE OrthoBASE, 

since the output is directly used by Stereo Analyst, and potential users of Stereo Analyst may already have that program. It is also possible to use IMAGINE OrthoBASE to derive missing information, such as computing the affine coefficients, without performing a full triangulation if the exterior orientation parameters are already known. However, these tasks will require more photogrammetric expertise than that of a novice GIS user, particularly if a full triangulation is required.

Measuring and Collecting Data. With oriented stereo models, Stereo Analyst provides the capability to measure and 

collect geographic information in three dimensions. In other words, geographic features in the stereo pair can be placed in the correct two-dimensional position. Height or elevation information is also part of the measurement. 3D information can be directly collected from imagery without the use of a digital terrain model. The program can also be effective while collecting ground points and break lines for the creation of digital terrain models. Tools are included to semi-automate the collection of features such as roads, buildings, parcels, and other items, including feature coding and attribution

The program also has the capability to work with existing GIS layers and themes. The data from these external sources can be registered to stereo digital imagery to perform quality control and editing. Also, data in 2D Shapefile format can be converted to 3D Shapefile format. A 3D reference source is associated with the vector file and subsequently superimposed on the digital stereo model.

Almost all aspects of using Stereo Analyst rely on the familiar concept of using a floating mark to measure parallax. The program offers many options for introducing a floating mark into the stereo model. These options allow control of color, shape, and movement. Even with these options, measurements using a floating mark still require skill and experience. To assist GIS users unfamiliar with stereo interpretation, the user interface can include embedded viewers that show the left and right portion of the stereo model in the vicinity of the floating mark. The user can employ these embedded views as a visual quality control tool to ensure that the floating mark rests on the feature being collected.

The 3D GIS data collected is stored as an ESRI 3D Shapefile, a format native to ArcView. They can also be exported as ASCII files, with a format that is documented in the User’s Guide.

Performance and Operations
Processing. The program uses large graphic images (two at a time), so it is very demanding of the hardware. The test system was a 450mhz Pentium II with 256MB of RAM and an ELSA GLoria XXL graphics card. General analytical performance was good. Some graphics intensive operations could take a while, depending on how much roaming or magnification was requested. Moving to adjacent and close areas of the image or making small changes in magnification occurs immediately (apparently these adjacent areas are buffered in video memory). More substantial changes require disk access and are directly affected by the hard drive speed. The use of pyramid layers for new imagery greatly speeds up redrawing when zooming. The higher-end graphics cards with large amounts of memory are also useful for speeding the imaging and graphics operations. (The test machine quickly became a favorite in the ESF Mapping Sciences Lab because the ELSA GLoria XXL graphics card was clearly more powerful than the more generic graphics cards in the other workstations.)

We encountered only one persistent image display bug while using the program. The problem occurred while using the option to mask out non-stereo areas of the image pairs. Using this option made it impossible to view the stereo overlap area. This effect happened for all of the stereo pairs. ERDAS technical support quickly responded to our inquiry and informed us that this was a documented problem with the specific graphics card being used. Upon a careful review of the release notes available for the software, we did find a warning about this particular setting. The problem was easily avoided by not choosing the option to “mask out non-stereo areas.” Occasionally, other glitches occurred in a way that could not be replicated. These types of glitches are familiar to any user of Windows operating systems, and are probably not the fault of the Stereo Analyst software.

Stereo Viewing Technique. We purposely compared the operations of Stereo Analyst while viewing with the color anaglyph mode to the same activities while using the quad buffered mode (“hardware stereo”). The hardware stereo approach did have advantages in terms of interpreting and measuring stereo features. Stereo features seem more easily distinguished, stereo interpretation is less tiring, the floating mark is more obvious, and it is possible to view color or color infrared imagery. The floating mark is more obvious because there is more flexibility when setting preferences for the cursor shape, size, and color. However, color anaglyph was still effective. The hardware stereo is recommended but not required.

Measurement and data extraction. Numerous observations were made while evaluating Stereo Analyst for the functions related to measurement and data extraction.

As with more traditional stereo viewing, measuring using the floating mark takes some skill and it will take a while to develop proficiency. Stereo Analyst’s capability to change the shape, size and color of the mark helps considerably. The reviewers found that it was important to vary the shape, size, and color of the mark depending on the imagery.

Stereo Analyst has a setting for Inverse Stereo that reverses the display of the left and right images. The User’s Guide states that Inverse Stereo “makes tall features appear shallow; shallow features appear tall.” So, ridges appear as valleys and valleys appear as ridges. In some cases, this effect makes some objects, such as trees, stand out more and is therefore useful. However, the effect was different in various stereo pairs or stereo viewing modes. Therefore, the method and purpose of inverse stereo can be somewhat confusing. For example, one stereo pair from the tutorial (“western_accuracy”) displayed in normal stereo with the Color Anaglyph mode but displayed in inverse stereo in Quad Buffered mode.

The actual measuring and collecting procedures worked well. These procedures involve many variations of mouse and key combinations, leading to some complexity and confusion to beginners and affecting efficiency. However, this problem should disappear as the user gains proficiency in using the software. It is very clear that the system works better when using a mouse with a scrolling wheel. For a user who will only occasionally use the routines, there will be a loss of efficiency while trying to remember the different procedures. The ability to digitize polylines in variable terrain (e.g. following sidewalks or roads) was effective. However, the end of the line at the floating mark did not follow the mark as it was moved up and down using the scrolling wheel. ERDAS technical support felt that this problem might be associated with the graphics card used on the test machine. This erratic behavior when floating the end of a polyline was a minor issue but could cause some frustration to users who are new to stereo interpretation. During the discussion, we learned that ERDAS has developed additional support for various programmable digitizing devices. These additional devices are scheduled to be available with version 1.2 to be released in September 2001.

The option for the terrain following cursor worked very well for data collection. While it would sometimes give false readings when moved in a uniform, featureless area (such as a parking lot or body of water), it greatly speeded things up. A new user will find it very helpful, although they will need to be careful and not automatically accept the data it provides. The embedded viewers of the left and right images are useful for verifying the results of the terrain following operation.

Stereo Analyst Version 1.1 includes a new feature for image texture extraction. The image textures associated with an extracted 3D model are automatically extracted and displayed in a 3D perspective window. The feature is useful while performing visual quality control and assurance during the collection of buildings and other 3D models. Texture extraction is also an interesting possibility for visual modeling.

The graphical user interface is fairly standard for the Windows world. However there are many icons whose graphics did not seem very intuitive. An explanation for each icon does appear at the bottom of the window when the cursor is placed on the icon. However, it would be much more helpful if a tip would appear at the icon when the cursor pauses over it.

Documentation
Stereo Analyst comes with a User’s Guide and online help. The User’s Guide has three main sections. Section 1 includes introductory material about Stereo Analyst, imaging, and photogrammetry. Section 2 includes five tutorial sessions that guide a new user through most of the program’s functionality using sample data sets. Section 3 contains reference material about installation and configuration, about resident capabilities, about ASCII output, and about the creation and display of oriented digital stereo models from external triangulation data. All three sections are effective and valuable.

The tutorial exercises are very detailed, easy to follow, and effective. The activities are suitable for a user without much background in photogrammetry (which is appropriate, since the User’s Guide is subtitled “Do-It-Yourself 3D Geographic Imaging”). In line with this theme, the guide also has an extensive introduction to basic photogrammetric concepts. To the credit of ERDAS, the reviewers found only one minor problem in the User’s Guide (the section dealing with “Measuring 3D Information”). In a section dealing with digitizing a polygon (pp. 161-163), the user is asked to digitize the “roof” of a “building that is not complex.” The problem is that the feature being digitized is not a building but is instead an ice arena with no roof. It took an experienced interpreter a while to figure out why he was having so much trouble digitizing this roof. A less experienced interpreter never realized the problem but experienced considerable frustration while trying to float the mark. ERDAS promises to include a correction in their Release Notes. We suggest that an Erratum also be distributed.

The online help is also useful, as it has a searchable index in addition to accessing the relevant topic while using the program. The installation also includes relevant help files and manuals in Adobe Acrobat Portable Document Format (pdf) from ERDAS IMAGINE if that program is not present. The documentation for the IMAGINE OrthoBASE module is particularly useful in working with oriented stereo models. The Release Notes that come with the software contain important and detailed information but have only minimal organization or indexing. Therefore, users may have a hard time finding information that applies to their specific situation.

Technical Support
ERDAS offers customer support by telephone, e-mail, and post. After an initial period, continued support is obtained using an annual support contract. The ERDAS web page contains more complete information about customer support. Only minimal support was required during this review, but that support was given freely and promptly by ERDAS personnel.

ERDAS Education Services provides a one day Stereo Analyst course discussing the principles and application of stereo viewing, 3D mensuration, 3D feature collection and attribution, stereo model creation and stereo interpretation techniques.

Installation
Stereo Analyst may be installed as a stand-alone product, as a module in ERDAS IMAGINE, or in association with ESRI’s ArcView. We successfully installed the software for all three situations. The stand-alone installation was performed on a computer without the proper graphics support. Therefore, Stereo Analyst would start properly but experienced problems upon trying to work with imagery. This stand-alone installation experience shows the importance of having the proper graphics hardware on the computer. Using Stereo Analyst in conjunction with one of the other programs provided some convenience during data setup or import/export. Installation and configuration information is contained in Section 3 of the User’s Guide.

Recommendations
Output formats are limited to 3D Shapefiles and simple ASCII files. The 3D Shapefiles are suitable for ESRI GIS products, certain ERDAS systems, and any others who have implemented the 3D Shapefile format. The ASCII output is a reasonable, but limited, attempt to make the output available to others. For most of these other users, special import routines will have to be available or developed in-house.

Overall the program was well implemented, easy to use, and able to fulfill its objectives of measuring and collecting geographic information. The most useful and powerful operations require that the stereo images have undergone absolute orientation. This orientation process requires preprocessing using external photogrammetry software systems (such as IMAGINE OrthoBASE) and is most easily performed by a person with some detailed knowledge of modern photogrammetry. Once oriented stereo models are provided, Stereo Analyst is suitable for someone experienced with GIS, but a novice to photogrammetry. Such a person will be able to start measuring 3D information quickly, but will need extensive experience with stereo interpretation before becoming highly effective.

Stereo Analyst is well suited for organizations that need to develop or verify geographic information. The program provides a well implemented and easy-to-use capability to measure 3D positions of vector and point features. Of particular note is the ability to directly account for terrain and scale variations while measuring 2D locations, heights and elevations. The only fundamental requirement is suitable stereo imagery that is in digital format and absolutely oriented. The effectiveness of the program will be significantly enhanced when users are proficient in stereo interpretation.
 

Related Information
The May 2000 issue of PE&RS contained a review of IMAGINE OrthoBASE.

All Other Worldwide Inquiries
+1 404-248-9000;  +1 404-248-9400 (fax)
info@erdas.com