Vendor
ERDAS, Inc.
2801 Buford Highway NE
Atlanta, Georgia USA 30329-2137
404-248-9000, 404-248-9400 (fax); www.erdas.com

Release Date: September 2001

Intel Pentium II or higher

64 MB of Memory (128 or more highly recommended)

CD-ROM drive for installation

Super VGA monitor - 800 x 600 x 256 colors (1024 x 768 x 64K recommended)

Centronics parallel port for hardware license key

Software

Windows NT 4.0 with service pack 6 or higher or Windows 2000 service pack 1

IMAGINE Advantageâ 8.5 (required, sold separately)

Stereo Analystâ (not required, allows for edits, sold separately)

Cost

IMAGINE OrthoBASE: $5,000

IMAGINE OrthoBASE Pro: $10,000.00 (includes IMAGINE OrthoBASE)

IMAGINE Advantage: $4,000.00

tereo Analyst: $3,000.00 
 
 
 

Summary
IMAGINE OrthoBASE Pro is an upgrade to IMAGINE OrthoBASE® that allows for fully automatic DEM (digital elevation model) extraction. It measures mass points by utilizing image-matching techniques. Extraction of DEM data is one of the major products of softcopy photogrammetry and was one of the first tasks to be automated. In previous years, ERDAS provided the UNIX-based module IMAGINE OrthoMAX™, which was supported up to ERDAS IMAGINE Version 8.3.

Now, with IMAGINE OrthoBASE Pro, a Windows NT- or Windows 2000-based solution is available that smoothly links into the other Windows-based modules. IMAGINE OrthoBASE provides tools for determination of exterior orientation parameters by automatic tie point collection and aerotriangulation (AT) for orthorectification. With the addition of DEM extraction, IMAGINE OrthoBASE Pro provides the missing link between AT and orthorectification by providing DEM data.

Both modules require a license for the ERDAS image processing software IMAGINE Advantage. Editing of the DEM data is possible either in IMAGINE Advantage or in Stereo Analyst. While IMAGINE Advantage includes a multitude of image processing tools, IMAGINE OrthoBASE, IMAGINE OrthoBASE Pro, and Stereo Analyst together make up what ERDAS calls their Photogrammetry Product Suite. For a review of IMAGINE OrthoBASE prior to IMAGINE OrthoBASE Pro, see PE&RS 5/2000. For a review of Stereo Analyst, see PE&RS 9/2001.

IMAGINE OrthoBASE Pro comes with a 30-day evaluation license, a set of tutorial data, and extensive documentation. As input for DEM extraction, it uses a variety of common image formats and either the AT results generated by IMAGINE OrthoBASE or exterior orientation parameters imported from external sources. It generates DEM data in the form of IMAGINE *.img raster files, TIFF raster files, ESRI Shapefiles, Terramodel TIN (Triangulated Irregular Network) files, or plain ASCII files. DEMs are generated either as individual DEMs per stereo model, or as a mosaicked DEM for the entire block. The software is designed to process large data sets in a GUI-based production environment and provides tools for quality analysis.

Features
Sample Data and Hardware
This evaluation is based upon the tutorial data set provided by ERDAS on the installation CD and on a data set provided by HJW GeoSpatial. The ERDAS imagery of Laguna Beach, CA was at a 1:40,000 scale, and was scanned at 50 microns. The HJW data set was at a 1:24,000 scale, and was scanned at 25 microns.

The latter data set consisted of five images of mountainous terrain and was relatively challenging mainly because it contained some heavily forested areas. In both cases, the exterior orientation parameters were known; therefore, no AT had to be performed in IMAGINE OrthoBASE. The evaluation was performed on a Pentium II 266 Mhz PC with 256 MB of memory and running Windows NT 4.0.

Operation
Block File
IMAGINE OrthoBASE appears as an icon on the main ERDAS IMAGINE toolbar. When IMAGINE OrthoBASE Pro is installed, the DEM extraction options appear under the existing dropdown menus of IMAGINE OrthoBASE. All menus are well-organized and guide the user easily through a logical sequence of processing steps.

Before starting the DEM extraction, the user has to set up an IMAGINE OrthoBASE block file containing at least two images. This binary *.blk file contains all necessary project information, such as interior and exterior orientation parameters, coordinate system, map projection and map units, camera data, input image file names and file paths, output DEM file names and file paths, as well as DEM processing parameters. Once the *.blk file has been defined, the images can be attached, and the image pyramid layers for hierarchical processing must be computed. Then, the AT can be performed, or exterior orientation parameters can be imported instead.

DEM Setup
The next step is to set up basic DEM extraction parameters. The basic parameters are output grid size, file name and file path of the output DEM, and DEM format. DEMs can be generated per stereo model, or as a merged DEM for the entire block. Possible data formats are:

• a regular grid in the form of an IMAGINE *.img file or TIFF file, derived from irregular mass points,
  
• iregular mass points in form of a Terramodel TIN file,
  
• iregular mass points in form of an ESRI 3D Shapefile, or
  
• iregular mass points stored in a plain ASCII file.

Advanced Parameters
If the user is satisfied with the default parameters, the process of DEM extraction can be started. Otherwise, a set of more advanced extraction parameters can be defined. The advanced properties window (see Figure 1) contains four tabs for General, Image Pair, Area Selection, and Accuracy parameter settings.

Under the General tab, the DEM can be output to a different map projection. An optional contour map can be generated in the form of an ESRI Shape file (*.shp) file, and a point status image that shows the quality of point matching can be produced.

Under the Image Pair tab (see Figure 1), the stereopairs can be selected. As the default, the system selects all image pairs with a certain user-defined minimum stereo overlap, e.g. 60%.

The Area Selection tab (see Figure 2) provides probably the most important options for advanced use. It allows for the definition of regions for DEM extraction with associated strategy parameters. As with the previous tab, this tab also allows display of the images.

Here, the user has the option to digitize regions in the mono image. An extraction strategy associated with each region can also be defined. One image can contain multiple regions (see Figure 2), each with a different strategy. Strategies can be selected from a set of predefined strategies, or by individually defining parameters (see Figure 3). Predefined strategies are for terrain types (e.g., flat, hilly, mountainous), surface filtering (e.g., none, medium, strong), object removal, etc. Custom strategies define parameters such as search window size, minimum correlation coefficient, etc.

Regions can also be used to exclude areas or define an elevation, e.g., for an area covered by water. The water areas can be delineated with the polygon measurement tool, or can be extracted automatically by region-growing techniques, which worked quite well with the provided sample data set. An elevation value for the lake can be entered, but this elevation must be measured outside IMAGINE OrthoBASE in Stereo Analyst, since a stereo viewing function is not integrated into IMAGINE OrthoBASE itself.

Finally, the Accuracy tab allows the user to specify reference data for a quality evaluation of the DEM (see Figure 4). This reference data can be either the tie points generated during the IMAGINE OrthoBASE AT process, an external DEM, or a set of check points, imported from an ASCII file. The documentation contains a thorough description of the statistical parameters used for accuracy assessment.

When the DEM extraction is finished, the strategy settings, processing times, and accuracy parameters can be reviewed in a report file. The quality of each extracted mass point based on the correlation value is stored together with the point coordinates.

Installation and Licensing
ERDAS IMAGINE and IMAGINE OrthoBASE Pro can be installed from CD. Installation is easy and straightforward, guided by Windows installation wizards. IMAGINE OrthoBASE Pro can be licensed on a single computer. In this case, the license is tied to the system ID of one machine. Alternatively, the license can be tied to a hardware key, which can be plugged into other machines as well. The license can also be installed as a floating license on a license server on the network. In addition to these two licensing options, IMAGINE OrthoBASE Pro will be fully functional with a 30-day evaluation license.

Documentation and Support
The IMAGINE OrthoBASE CD contains a 548-page user’s guide in *.pdf format. This extensive documentation covers a wide range of topics¾from the basics and history of photogrammetry for the novice user to the methods of automated DEM extraction for the more advanced user. It contains a detailed description on how the accuracy parameters of the DEM extraction report are computed as well as several Tour Guides, which guide the user step-by-step through the tutorial data sets included on the distribution CD. It also contains valuable tips, such as how to manipulate the DEM extraction strategy parameters, etc.

ERDAS’ support by e-mail and phone has always been good, and the development team has usually been very responsive to customer needs. ERDAS also offers training classes for ERDAS IMAGINE and for IMAGINE OrthoBASE Pro.

Conclusions
With IMAGINE OrthoBASE Pro, ERDAS has filled a gap in the functionality of the original IMAGINE OrthoBASE product. IMAGINE OrthoBASE Pro now can perform all tasks from determining the exterior orientation over DEM extraction to orthorectification. Like IMAGINE OrthoBASE, the DEM extraction part of IMAGINE OrthoBASE Pro is well-designed and intuitive to use. Menus make it easy to follow a logical workflow, and a table-like frame editor helps to keep track of which steps have been completed with red or green boxes.

One particular strength of IMAGINE OrthoBASE Pro is its ability to process large data sets in a well-organized manner. The user does not have to select much more than a directory and a minimum percent of overlap to start the extraction of DEM data from all stereo image pairs in the directory with the minimum overlap. This is unlike some other products, which often require a lot of mouse click operations to trigger a process.

Besides the ease of operation for a novice user, the system provides a lot of flexibility for the advanced user. Although the software is clearly aimed at the non-specialist and the GIS user who needs to generate his or her own DEM data, fine-tuning the extraction parameters to obtain optimal results requires some understanding of the image matching process as well as some experience with the software.

IMAGINE OrthoBASE Pro runs relatively fast and has minimum requirements for the hardware. Although it is able to generate a variety of output data formats, editing the data requires either *.img format for area editing in ERDAS IMAGINE, or Shapefile format for mass point editing in Stereo Analyst. Stereo Analyst has not been part of this review since it was reviewed for an earlier edition of PE&RS. Editing the data in Stereo Analyst naturally has much higher hardware requirements.

The focus of ERDAS IMAGINE has been to provide image analysis tools for photogrammetry, remote sensing, and mapping. Now, the ERDAS Photogrammetry Product Suite, consisting of IMAGINE OrthoBASE Pro and Stereo Analyst, can be linked into ERDAS IMAGINE at a relatively low cost. Today, the cost of a softcopy workstation ranges from roughly $5,000 to $80,000. IMAGINE OrthoBASE Pro can be obtained for the fraction of the cost of a high-end photogrammetric workstation.

Some drawbacks of the software include limitations of the editing capabilities. Also, the algorithm could be expanded to include breakline extraction in addition to mass point extraction. Additionally, in low textured areas or forest areas, points may have to be added in Stereo Analyst.

Overall, IMAGINE OrthoBASE Pro is a very useful production tool to provide DEM data. This data can be used as one of the main layers of any GIS, a prerequisite for orthorectification, or as input for generating fly-throughs in IMAGINE VirtualGISä. IMAGINE OrthoBASE Pro provides comprehensive functionality at a reasonable cost.

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