Digital Color Orthophoto Maps

Apollo Teng, DIST

(This article appears in GIS News, Vol. 5 No. 4, July 1999)


As part of the MC:MAPS consortium, DIST GIS Team managed the ‘pre-mapping’ project during FY 93 – 96. The four phase-project covered the whole County. The project entailed the acquisition of (black and white) aerial photographs, densification of the ground control points, and performing aerial triangulation and establishing/calculating precise coordinates for 9 control points for each diapositive. These tasks were necessary so that M-NCPPC's mapping contractors could stereo-compile a set of precise (at 1"=200’ scale) planimetric base map layers.

As a by-product of the DIST project, DPW&T (Transportation Management Center) contributed fund to have the DIST contractor produce digital orthophoto files (DOF’s). Basically, DOF is a digital photo-map with distortions corrected. TMC’s interest in acquiring the DOF’s lied on using it as background map for its traffic management system. Througout the four phases, a total of 312 map sheets of DOF’s were created. (With the WSSC grid system, the County is divided into 668 map sheets.) In the subsequent two years, M-NCPPC’s base map contractor also created 200+ map sheets of DOF’s.

These image files have been widely used among the various agencies within the County. However, two major shortcomings that prompted us to look for something better. The first is the outdatedness. The north and west part of the County was flown in 1994. Yet, much of the northern part of the County has been experiencing rapid growth. The second is that black and white (ortho)photos are less conducive to feature recognition as compared to the color ones. Furthermore, more than 100 map sheets along the County borders were without the DOF’s.

Around the same time, the State Department of Natural Resources (DNR) created Color Infra-red (CIR) Digital Orthophoto Quarter Quad (DOQQ) covering the whole state. DIST purchased (with minimal cost) a copy covering the County. Being at a scale of 1"=1,000’, the accuracy is at + or – 20 feet. The images do not overlay with MC:MAPS base maps (1"=200’) well. The images are outdated. CIR is not true color. These limitations rendered the data set of limited use.

Need for the Up-to-date Color Orthophoto Files

To alleviate the problems associated with the existing set of black and white image files, DIST GIS Team started looking for ways of acquiring one that would meet the need. The following departments can readily bemefit from a set of up-to-date color orthophoto fiels:

DEP: To be used for stream water protection, reforestation planning, storm water retention pond management.

Permitting Services: To be used for existing condition examination, inspection route planning

Community Use of Public Facilities: To be used for school ground, ball field inventory

DPWT Office of Project Development: for various transportation improvement projects

DPWT Engineering Services: for planning various sidewalk, bridge improvement and neighborhood revitalization projects

DPWT Facilities & Services: for planning facilities construction, expansion, or improvement projects

DPWT Transportation Management: as the background map for all traffic control programs

DHCA: for planning community and neighborhood revitalization projects.

Office of County Executive: for planning traffic control for major events, First Night, Golf Tournament, etc.

County Council: for encroachment investigation, road retirement approval, etc.

Recreation: for identifying recreational facilities.

Police: search and rescue planning

In early 1998, representative from VARGIS LLC of Herndon, Virginia visited DIST GIS Team and offered such products based on March 1998 photography of the region.

Funding the Purchase

The organizational perpetual license fee for the VARGIS products for Montgomery County was set at $71,485. Additional $11,000 was required to tile the data set by WSSC grids (instead of the standard VARGIS grids for the metropolitan area). Thus, the total fund required was $82,485.

DIST GIS approached several departments/divisions for contribution of funds. The reaction was very enthusiastic. Commitments for a total of $82,485 were secured from six departments/divisions in one-week time. They are: DEP, Community Use of Public Facilities, DPWT (Transportation Management Center and Director’s Office), DPS, and DHCA. Attempts to obtain additional fund from MC:MAPS agencies for the purpose of adding Unix disk drives were not successful.

Contract (# 9341000006AA) with VARGIS LLC was signed on January 6, 1999. The Attachment C: DIST License Addendum extends the rights (and restrictions) to other GeoMaP Consortium members, namely Maryland-National Park and Planning Commission (M-NCPPC), the Washington Suburban Sanitary Commission (WSSC) and Prince George's County. The use of the products is for official agency projects performed by agency employees or consultants. The whole set of 129 product CD’s were received on May 3, 1999.

Making Data Accessible to the Users

The 3-foot resolution version of the 668 TIF files were uploaded to the GIS Database Server (from 17 CD-ROM’s). Users were notified of the availability on May 5.

There were not spaces needed for uploading the 1-foot images (88 MB x 668). The popular image compression software MrSID Compress (Publisher version) was acquired ($4,000) in early June. Each map sheet (22 MB x 4 sub-sheets = 88 MB) was compressed (20:1) and merged into slightly over 4 MB in size. The availability of this series was announced on June 22, 1999.

In late May, OMB approved DIST request to convert some of the excess salary fund (due to delay in filling up some DIST positions) to capital equipment purchases. An order for nine 9 GB disk drives ($18,315) was issued (making the total investment at $104,800). This addition should provide enough on-line space for storing the whole set of un-compressed images. These expansion-disks arrived on July 16, 1999.

The March 2000 Version

VARGIS plans to fly the region again next spring. They offered the County same pricing ($82,485) if ordered before December 31, 1999. DIST GIS Team will ‘pass around the hat’ again soon!