Technical Information

Digitizing the Collection

Data page images

The data pages in the Historic American Building Survey/Historic American Engineering Record/Historic American Landscapes Survey (HABS/HAER/HALS) collections were captured at the Library as digital facsimile images by Systems Integration Group of Lanham, Maryland, using a Minolta PS3000 scanner, a device with an overhead camera design. A series of tests established that the typescripts (from the early period) and computer printouts (from later years) would be fully legible if reproduced as bitonal images with a spatial resolution of 200 dpi and this specification has been applied to the scanning. At 200 dpi, a typical 8.5x11-inch page yields an image of 1700x2200 pixels. The resulting images are saved in the TIFF (Tagged Image File Format, version 6.0) format and compressed with the lossless ITU Group IV algorithm and a typical file is about 35 kilobytes. These are referred to as the archival or master images.

The Library's online presentation of the data pages employs a "page turner" or "electronic binder" that permits the user to navigate the data pages, turning to the next or a previous image, or selecting an image by number. The inline service images displayed in the browser are GIF images with a spatial resolution of about 495x640 pixels; a hyperlink on the image provides access to the master image. Using the master images as a source, the Library produces the inline service images in a batch mode that includes four steps:

(1) adding tonality (transform the bitonal image into a 16-color grayscale image),

(2) blurring the image to soften the edges of the characters;

(3) reducing the image from about 1700x2200 pixels to 495x640 pixels;

(4) applying an image enhancement algorithm to suppress the speckling that sometimes results from the addition of tones and reduction in size, and;

(5) sharpening the image to restore some clarity to the characters.

Some of the data pages consist of photo captions associated with particular HABS/HAER/HALS surveys. After capturing images of the caption pages, Systems Integration Group sent them to a subcontractor for rekeying. The rekeyed texts did not receive any coding or markup. After delivery to the Library, the rekeyed lists were formatted to create a small dataset for the survey to which they pertain. These datasets are the source for the captions displayed with the images for some surveys. (For other surveys, the HABS/HAER project recorded the captions on photo mount cards but did not include them with the data pages. The photo mount cards are not being scanned in the current round of work but the Library plans to rekey and present these captions in the future.)

Specifications for the data pages

Archival or master images

Spatial resolution:

200 dpi

Tonal (pixel-depth) resolution:

1 bit-per-pixel

File format:

Tagged Image File Format (TIFF) ver. 6.0

Compression:

ITU Group IV

Inline service images

Photographs

The original negatives and color transparencies in the Historic American Building Survey/Historic American Engineering Record/Historic American Landscapes Survey (HABS/HAER/HALS) collections consist primarily of 4x5-, 5x7-, and 8x10-inch cut film exposed in view cameras. By scanning the original negatives and transparencies, higher quality digital images can be produced than would be obtained by scanning the collection's prints, which are one generation removed from the original exposure and, especially for the early years, have faded or deteriorated.

The HABS/HAER/HALS photographs are one of the first American Memory pictorial collections whose images have been produced by direct capture with a digital camera at high levels of spatial resolution. Most of the online collections of photographs and prints for which production began prior to 1998 were produced via photographic intermediates and, with the exception of the Panoramic Photographs, at lower levels of resolution.

The Library does not call the archival or master digital images of the HABS/HAER/HALS photographs "preservation copies," but these digital images will play the same role with respect to the originals as copies made on photographic film have in the past.

The digital images are being produced by JJT Incorporated of Austin, Texas, using their new overhead-capture MARC II digital camera. For the HABS/HAER/HALS photographs, the initial capture uses a 7000x7000 pixel matrix at 12 bits-per-pixel for grayscale or 36 bits for color. Image processing is then executed at 16-bits-per-pixel for grayscale or 48 bits for color. The bit depth is increased because having two even bytes per channel facilitates post-processing. For the archival or master image as saved, the spatial resolution is being reduced to 5000 pixels on the long side (with the short side scaled in proportion) and the tonal resolution reduced to 8 bits per pixel for black and white items and 24 bits for color. JJT's post- processing step also creates the three service images that complete the set of four delivered to the Library.

The digital images are intended to serve research, particularly research pertaining to historic buildings and engineering structures. Although aesthetic considerations played a role in determining the look of the digital images, it was also important to retain highlight and shadow detail so the maximum amount of information can be derived from the image. This means that the images are made at lower contrast than might otherwise be the case. Through the years, the contact prints served to researchers in the Prints and Photographs Reading Room were made in a similar manner, i.e., at a contrast level that reveals as much information as possible, and with little or no burning and dodging. Users who require digital images that are cropped, have higher contrast, or are improved by local adjustments, should download the archival versions and manipulate them in graphic arts software.

Specifications for the photographs

Uncompressed Archival Images

Spatial resolution:

About 5,000 pixels on the long side with the short side falling where it may. This spatial resolution was employed regardless of the size of the original negative or transparency.

Tonal (pixel-depth) resolution:

Grayscale:

8 bits per pixel

Color:

24 bits-per-pixel

File format:

Tagged Image File Format (TIFF) ver. 6.0

Compression:

None

Compressed Service Images

Spatial resolution:

Medium resolution:

640 pixels on the long side with the short side falling where it may.

Higher resolution:

1024 pixels on the long side with the short side falling where it may.

Tonal (pixel-depth) resolution:

Grayscale:

8 bits per pixel

Color:

24 bits-per-pixel

File format:

JFIF (JPEG File Interchange Format)

Compression:

Medium resolution:

JPEG at a quality setting that yields an average compression of 15:1.

Higher resolution:

JPEG at a quality setting that yields an average compression of 8:1.

Thumbnail Images

Spatial resolution:

About 150 pixels on the long side with the short side falling where it may.

Tonal (pixel-depth) resolution:

8 bits-per-pixel; palettes optimized (adaptive palettes) for each image.

File formats:

Archived copy:

Tagged Image File Format (TIFF)

Online copy:

GIF (Graphics Interchange Format) format

Compression:

Archived copy:

uncompressed Online copy: compression native to the GIF format.

Measured drawings

In 2000-2001, a contractor, JJT, Inc., of Austin, Texas, used an Océ 9800 scanner to scan the 55,000 original measured drawings that the HABS/HAER/HALS programs of the National Park Service had sent to the Library of Congress since 1933. In 2004-2005 the Library of Congress Digital Scan Center used an IDEAL/Contex Premier TX36 scanner to scan approximately 4,800 additional drawings that had been transmitted to the Library since 2000. Since 2004, the National Park Service has included digital images of the architectural drawings as part of their transmittals to the Library.

HABS/HAER drawings are either B (18x24-inch), D (24x36-inch) or E (34x44-inch) size. The drawings are scanned at 400 dpi. Since the originals are line drawings, the images are bitonal and compressed with the lossless ITU Group IV algorithm. Typical compressed file sizes are about 700-800 kilobytes. Uncompressed file sizes range from 9 MB (B size) to 30 MB (E size).

After the master images are delivered, the Library produces three service images, one for the initial, "contact sheet" inline display in the browser, a second larger reference view, and the third compressed service image for printing on a desktop printer. The browser images are sized to fit most screen displays, while the printing image is sized to fit on a sheet of typing paper. The master image can be used to print the drawing at its original size. The browser images are produced in a process like that used to produce the "page turner" images of the data pages, described above:transforming to grayscale, blurring, reducing in size, and applying an image enhancement algorithm to suppress the speckling that sometimes results from the addition of tones and reduction in size, and sharpening.

The printing images are produced by reducing the spatial resolution of the archival images and recompressing with the ITU Group IV algorithm.

Specifications for the measured drawings

Archival or master or images

Spatial resolution:

400 dpi

Tonal (pixel-depth) resolution:

1 bit-per-pixel

File format:

Tagged Image File Format (TIFF) ver. 6.0

Compression:

ITU Group IV

Inline service images

Spatial resolution:

About 640x480 pixels

Tonal (pixel-depth) resolution:

4 bits-per-pixel

File format:

GIF (Graphics Interchange Format) format Printing service images

Spatial resolution:

300 dpi; scaled to about 2550x3300 pixels

Tonal (pixel-depth) resolution:

1 bit-per-pixel

File format:

Tagged Image File Format (TIFF) ver. 6.0

Compression:

ITU Group IV

Describing the Collection

The HABS/HAER/HALS Catalog has been compiled and maintained by the HABS/HAER/HALS office of the National Park Service since 1982. The Library of Congress Prints and Photographs Division has arranged for the data to be displayed in categories comparable to other Prints and Photographs collections.

The following comments explain the data categories and cataloging guidelines. They also point out which catalog record information is most useful for citing or ordering pictorial materials in research notes or publications. Since the original information accompanying a picture sometimes can be inaccurate, the Prints and Photographs Division is always glad to hear from researchers who have additional or better information. Please be aware that it usually takes several weeks to confirm information and make changes to catalog records. The on-line LC-HHH Catalog is updated with additions, corrections and revisions quarterly.

For general information about cataloging pictorial materials, see the Visual Materials: Processing & Cataloging Bibliography .

TITLE. The title is a compilation of elements from the National Park Service's HABS/HAER/HALS Database. In order, these are i) the historic name of the structure as recorded by HABS/HAER/HALS; ii) street address (urban: the number and street on which the structure is located, rural: the nearest public road); iii) city, town or vicinity in which the structure is located; iv) county or counties in which structure is located; v) state postal abbreviation for the state in which the structure is located.

Historic names can include building/structure uses (e.g., Chesterfield Blacksmith Shop); personal names that have become part of building names, listed by last name, first name, and descriptive building type (Guthrie, Woodie, Birthplace); historic districts (Savannah Victorian Historic District); and complexes, followed by the individual structure name (Wright-Patterson Air Force Base, Hangar No. 1.)

OTHER TITLE or ALTERNATE TITLE. Additional titles/names by which the building/structure is known.

MEDIUM. Describes the quantity and the physical properties of the original documentation given in terms of the following broad categories:

• "measured drawings" = architectural measured drawings
• "b&w photos" = black-and-white photographs
• "data pages" = written histories
• "photo caption page" = typed captions for the photographs
• "color transparency" = color transparencies

The description is also a reminder that the physical characteristics of the original object are quite different from a digital reproduction on a computer screen. Note: The dimensions of the documentation formats are not available in the catalog record.

(Search Tip: The term "drawings" may be used to isolate catalog records for surveys containing measured drawings, although the terms "photos", "page(s)" and "color transparencies" are not searchable terms in the catalog.)

CALL NUMBER. This string of letters and numbers is used to locate the material served at the Library of Congress. The call number is formulated as follows:

HABS, RI, 16-ASH, 101-

The collection ("HABS"), the state ("RI," for Rhode Island), the county ("16-" for Kramer, the sixteenth county alphabetized in Rhode Island), the city ("ASH," being the abbreviation for the town of Ashland), and the unique number assigned for the building within that town ("101-".)

The phrase "Unprocessed Items..." indicates that certain documentation is either at the HABS/HAER/HALS office, or has not yet been cataloged by the Library of Congress and is therefore generally unavailable to the public.

DATE or CREATED/PUBLISHED. This date refers only to the date that the respective collections began: 1933 for HABS, 1969 for HAER, and 2000 for HALS.

The date the documentation for a survey was compiled is not noted in the database, nor can a specific date be isolated, as documentation is often compiled over a period of months, and sometimes years. To find the specific date for a survey, consult the digital images for data pages where the compilers name and a date are often included. To find the specific date for a photograph consult the digital images's of the photo caption pages which often include the date and name of the photographer. To find the specific date for a measured drawing, consult the digital images for measured drawings where the delineators name and a date may be included. Dates for the structures, if available, are given in the NOTES field.

NOTES. The notes are a compilation of elements from the National Park Service's HABS/HAER/HALS Database. These include a project's Survey Number (e.g., HABS CT-22); Building/Structure dates and Building/Structure Use; the location of documentation not yet sent to the Library of Congress; and the existence of background field note material.

The project's Survey Number identifies either the HABS, HAER, or HALS program designation, the two letter state abbreviation as used by the Postal Service, and the structure or site number representing the approximate numerical sequence in which the structure was recorded by HABS, HAER or HALS in that state, i.e., HABS [program] CT- [state] 22 [approximate sequence recorded.] For complexes, an identifier of A, B, C, etc., is given for structures associated with the overall record entry.

The abbreviation ca., usually found in notes about dates, means circa and indicates a date that is approximate within several years.

SUBJECTS. The Historic American Buildings Survey/Historic American Engineering Record/Historic American Landscapes Survey uses an uncontrolled indexing vocabulary where different terms, such as "Car dealership" and "Auto dealership," are sometimes used for the same subject, because the material being cataloged used those different terms. The subject headings are actually the words in the Building/Structure Use Note field, and less than half of the records have entries in that field.

(Search Tip: Also use the "Keyword" search in the LC-HHH Catalog for a more complete subject search.)

The place names are expressed as hierarchical geographic "strings" to allow keyword access to states and territories as well as counties and cities; for example, "Virgin Islands-St. Croix County-- Christiansted."

RELATED NAMES. When multiple people or corporate bodies contributed to a building/structure, their names may be listed as related, or added, entries. Related names include, for instance, the architect(s), owner, builder, etc., when this information was recorded by the creators of the catalog records. If related names have not have been recorded in the catalog record, they may be found in the data pages (written histories.) (See DATE or CREATED/PUBLISHED for information on documentation historians or other compilers, photographers, and delineators.)

REPRODUCTION NUMBER. The reproduction order number is usually the Call Number or, for full-size measured drawings, the Survey Number. See How to Order for information needed to order photographs, measured drawings or data pages. Reminder: Also consult the note field when material has the call number "Unprocessed" - the material may not yet be at the Library of Congress.

COLLECTION. The title of the collection associates each item with its source and is useful to include in bibliographic citations.

REPOSITORY. The name of the institution and division that have custody of the original work. This information can help you locate or em the original.

CARD #. The control number, or card number, for each catalog record is a unique identification number. It can be used to do a quick number search when you want to see a specific record without repeating a long keyword or subject search. However, only some online catalogs provide an index by this number. (Available in the Prints & Photographs Online Catalog only.)