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Working With Image Attachments in PLS-CADD

ECW and JPEG 2000 support were significantly improved in PLS-CADD version 11.01 and the following comments are applicable to that version and newer.

The use of raster images whether aerial photographs or scanned Plan & Profile (P&P) sheets, can lead to more pleasant and productive PLS-CADD work. Images are attached in PLS-CADD using the Drafting/Attachments/Attachment Manager command. They can be attached to the plan (for aerial images or USGS DOQs) or the profile (for scanned P&P sheets) views. PLS-CADD's support for raster images is summarized in the following table:

Image Formats Supported By PLS-CADD
Format File Extension Compression Scanned P&P Sheets Aerial Photographs Georeferencing
Windows Bitmap .bmp Poor (lossless) OK to use OK to use TIFF World File
Tagged Image File Format .tif Good (lossless) Best choice OK to use TIFF World File or GeoTIFF
JPEG .jpg Very good (lossy) Not recommended OK to use TIFF World File or Built in GPS
Enhanced Compressed Wavelet .ecw Excellent (lossy) Not recommended Great choice TIFF World File or Built in
JPEG 2000 .jp2 Excellent (lossy and lossless options) OK (lossless only) Great choice TIFF World File or Built in

The Windows Bitmap format (BMP) can be thought of as the jack of all trades - it can do everything, but doesn't do anything particularly well. The files have optional compression, but since the compression algorithm is crude the file size tends to be large relative to a comparable TIFF, ECW or JP2 file. The primary advantage of a BMP file is that it has near universal support due to its simplicity. BMP files do not contain georeferencing information.

Tagged Image File Format (TIFF) is an open industry standard format much as the BMP format is. The TIFF format is almost universally supported. Like the BMP format it features lossless compression and is a workable solution for both aerial imagery and scanned P&P sheets. However, due to its more sophisticated representation of images it can frequently achieve vastly better compression than a BMP file. Specifically, TIFF files are excellent for representing scanned P&P sheets and should always be used for them. GeoTIFF is a superset of the TIFF format that contains georeferencing information, PLS-CADD will read this information if present and automatically place your image in the correct location in your project without you needing a corresponding TIFF World File (*.tfw).

Joint Photographic Experts Group (JPEG) is an open industry standard format like TIFF and like TIFF, it enjoys universal support. JPEG uses lossy compression and so is an acceptable choice for aerial imagery. Like TIFF files, JPEG files may be georeferenced via a World File (*.jfw for JPEG files). Unlike TIFF files, JPEG files may contain the GPS coordinates of the camera when the picture was taken. If the project has a coordinate system defined and the image contains GPS coordinates then PLS-CADD will locate the image based on those coordinates. While not georeferencing per se, this can be useful nonetheless. For example, PLS-CADD will automatically place pictures of structures near the structure they show.

The Enhanced Compressed Wavelet format (ECW) is a proprietary format developed by ERDAS. Earth Resource Mapping provides plug-ins for a wide variety of software so support for the ECW format is quite common. ECW features an excellent compression ratio that typically achieves at least a 20:1 reduction in file size. However, it is primarily intended for aerial imagery and the lossy compression it uses can render scanned P&P sheets illegible, so it should only be used for aerial imagery. Note that a USGS DOQ is more like aerial imagery than like a scanned P&P sheet so ECW files can be used for DOQs as well. The Attachment Manager in PLS-CADD can convert TIFF or BMP files to the ECW format for you. ECW files can contain georeferencing information; if they have this information PLS-CADD will read it and automatically place your image in the correct location in your project.

JPEG 2000 (JP2) is very similar to ECW in that it uses a sophisticated compression algorithm that provides excellent compression ratios, has built in georeferencing and works well for aerial imagery. JP2 files can be used in any situation you would use an ECW file, but because it has a lossless option it may also be used for scanned P&P sheets. Unlike the proprietary ECW, JPEG 2000 is an open standard (ISO/IEC 15444-1); however, support for it is not yet as widespread as ECW. PLS-CADD can create JPEG 2000 files via the "Compress" button in the Attachment Manager.

ECW or JPEG 2000? There is no clear cut winner when comparing these file formats. For the same image data, ECW files will decode faster so that you will have less delay before seeing your images, but JP2 files will tend to be smaller. JPEG 2000 is an open standard, but ECW is more widely supported. Whichever you choose you will benefit from PLS-CADD's sophisticated automatic memory management, background image loading and tiling capabilities.

Georeferencing

As mentioned previously GeoTIFF, ECW and JP2 formats can contain georeferencing information directly in the image. If this information is present, PLS-CADD will use it to locate your image. In addition, all images can have a corresponding TIFF World File which PLS-CADD can read. In the event an image has both built in georeferencing and a TIFF World File then PLS-CADD will ask you which one to use. The TIFF World File has the same file name as the image, but uses a ".tfw", ".bfw", ".jgw", ".j2w", ".eww" or ".bmw" extension. This file contains six lines of information used to locate your image. These six lines are:

1.	X scale
2.	Rotation information
3.	Rotation information
4.	Y scale (as a negative number)
5.	The x coordinate for the center of the upper left pixel
6.	The y coordinate for the center of the upper left pixel

An example World File follows: 
1
0
0
-1
1000.01
1000000.02

Memory Management

Working with images is a joy right up until your computer runs out of memory. When the sum of the memory required for your images exceeds the memory available in your computer than everything will slow to a crawl. PLS-CADD has several strategies for avoiding this unhappy occurrence, which one you employ depends on what type of image you are working with.

For scanned P&P sheets you should use the Hide button in the Attachment Manager to hide each image when you are done digitizing it. Hidden images do not consume memory, so you can have as many of them attached as you like. This is preferable to the Detach command as that will completely disassociate the image from your project resulting in the loss of any calibration information. It is very unlikely that you would calibrate the image exactly the same way should you ever need to reattach it, so you should never detach a scanned P&P sheet.

For aerial photographs PLS-CADD automatically manages memory. It dynamically varies the resolution of each image and even the resolution of different regions within an image. The resolution used is a function of the available memory, project size, original image resolution and which images are visible on screen. PLS-CADD adjusts resolution automatically to optimize display and plot quality. You may occasionally experience short delays when panning and zooming as PLS-CADD re-optimizes image resolution in response to new display scale and extents. This dynamic resampling technique works best with ECW and JP2 image formats as they natively support multi-resolution imagery. It can also work with BMP, JPEG and TIFF images but it will be much slower as they do not natively support multi-resolution imagery.

Further Reading

Our technical note that describes How to Digitize Existing Plan and Profile Drawings (includes a video demonstrating the process) can be very helpful when you start digitizing P&P sheets.


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