Using GIS and Remote Sensing to Streamline Loudoun County’s Base Map Update Process

Loudoun County, Virginia located within the Washington, D.C. metropolitan area, is approximately 520 square miles and has been one of the most-affluent and fastest growing U.S. counties for the last decade. The County’s eastern portion is developing rapidly, while many western areas remain rural. Recently, however, development has expanded into these historically rural areas.

Three images of the same location show the growth of Loudoun County from 1978-2015

Escalating Population of Loudoun County from 2001-2021

Data Source: US Census Bureau, Loudoun County Department of Finance and Budget

The rapidly changing landscape and growing population have necessitated aggressive maintenance of many spatial data sets, particularly land records and the County’s base map. A base map typically refers to a representation of geographic features that are visible on the ground in an aerial image. They provide the foundation or reference upon which other data layers can be developed or mapped, and are the foundation of the County’s geographic information system (GIS).

The base map update is an ongoing project that entails maintaining and updating the GIS base map layers, including buildings and roads, cultural features, hydrography and forest cover, contours and spot elevations. The GIS base map is uploaded to the County GIS and distributed to the users on an annual basis.

Ground features are captured and attributed through the County’s base map maintenance services contract, allowing the County to streamline its base map development process and maximize the use of this valuable resource.

Using an airplane-mounted Microsoft/Vexcel Ultracam digital camera system, Loudoun County is flown each spring to capture new imagery and photogrammetric data. Aerial imagery is collected with full stereoscopic coverage, and flights are conducted when the following conditions are met:

There’s sufficient sun angle: at least 30 degrees above the horizon
The air is burdened with minimal smoke, smog, haze, fog, dust or other obscuring phenomena
The ground is free of snow and leaf conditions that may obscure ground features
No unusual flooding conditions are evident

Once imagery acquisition is done, aerotriangulation is performed using ground control points and an airborne Global Positioning System. Aerotriangulation is a method of determining the coordinates of points of a terrain on the basis of aerial photographs. Once aerotriangulation is performed successfully, a digital terrain model (DTM) is compiled for ortho generation and for planimetric and topography compilation.

Planimetric features in geography are those features that are independent of elevation, such as roads and building footprints. They are represented on two-dimensional maps as they are seen from the air. These features are often digitized from rectified aerial imagery into data layers that can be used in analysis and cartographic outputs.

Authors: Kenneth E. Foote and Margaret Lynch

The topogaphic features are the relief features or surface configuration of an area such as contours and spot elevation points. These features are mostly generated by some automated process.

Source: Leroy R. Grumman Cadet Squadron

Using the new aerial photography, features are updated in a 3-D soft-copy stereo environment at a scale of 1:2,400. After compilation, data are exported to an ArcGIS geodatabase feature class. All base map data have been tested and meet NSSDA and ASPRS accuracy requirements for 1:2400 scale maps.

Orthoimagery consists of aerial images that combine the visual attributes of an aerial photograph and the spatial accuracy and reliability of a planimetric map. Orthophotographs are important to Loudoun County, as they’re used as a reference when mapping all other data. As part of the County’s base map update process, digital orthophotos have been produced since 2002 and are available to GIS users through the County’s Enterprise geodatabase using Esri’s ArcGIS desktop software and in an Aerial Archive web application as an added resource. Orthoimagery is used as a backdrop with other data layers on top. Datasets such as parcels, addresses, street centerline, zoning and planning districts often rely on imagery datasets.

Orthoimagery is the foundation of the County’s GIS. Various departments depend on the County’s orthoimagery and base map data to produce their own maps and analyses. Loudoun's historical imagery can be found in the Aerial Archive. The latest image is always used in WebLogis- Loudoun's online mapping system.

One-foot countywide digital orthos are available in the following formats:

Uncompressed GeoTIFF covering 5,000- x 5,000-foot tiles each.
Single uncompressed GeoTIFF comprising the entire county.
Compressed MrSID covering 5,000- x 5,000-foot tiles each.
Single compressed MrSID comprising the entire county.

Base map data layers have features that typically fit into one of three general types: polygon, line or point. Base map layers include buildings, road casings, drainage, water bodies, forest, tree points, topography, spot elevations, miscellaneous polygons (recreational fields, swimming pools, quarries, golf course features, etc), miscellaneous points (manholes, poles, hydrants and towers, etc.), and miscellaneous arcs (sidewalks, culverts, bridges, trails, fences etc.).

Each base map feature has an update date field associated with it which shows the year when that particular feature was last updated. Base map data is obtainable from OMAGI's (Office of Mapping and Geographic Information), Public Information Counter. Updated DTM is available on request as well. All data products are georeferenced in the Virginia State Plane (North), Zone 4501, datum NAD83 HARN, vertical datum NAVD88, with units of U.S. Survey feet. Metadata that meet Federal Geographic Data Committee standards are available for all base map layers.

The County’s base map and most of the other corporate spatial data are developed and are maintained at a scale of 1:2,400. The base map is a conventional photogrammetric product that is maintained annually from aerial photography. The original base map was developed from aerial photography between 1978 and 1985.

The Countywide remapping project, conducted in two phases, was undertaken to produce the current data set. Phase I, using 2002 Virginia Base Mapping Program digital scanned imagery and Phase II, using 2004 scanned aerial photography, comprises the initial re-map effort. The initial project was completed in 2005 and now undergoes annual updates.

The first annual update, Phase III, was derived from 2005 imagery and completed in fall 2006. The second annual update, Phase IV, was derived from 2007 imagery and completed in spring 2007. Phase V of the base map updates was completed in late 2008. Most recent updates were derived from 2023 imagery and completed in summer 2024. 2024 updates are underway and anticipated to be completed by summer 2025.

The base map maintenance program has resulted in a model supporting emergency response, assessment, land use planning, transportation and other key business functions of the County.
Updating large, contiguous areas has resulted in consistent and timely data for a variety of applications. Data sets such as parcels, addresses, street centerlines, and zoning and planning districts often rely on base map delineations.
Loudoun County has a seamless, countywide base map geodatabase stored at a central, secure location and can be assessed in various ways across many county departments and by the public.
Uniformity of the base map data meet the expectations of the internal customers and the end users and can be used as a reference when mapping all other data.
Analysts, planners, business people and other decision makers use base map data to discover and demonstrate spatial relationships, making the base map a valuable tool to explore management and policy alternatives.
Base map data is critical to developers/builders when applying for building permits. WebLogis- Loudoun County’s online mapping system provides access to base map layers.
Using Loudoun's GIS Open Data resource anyone can download and use base map data at no cost. Using this data, users can share their insights with their community or build new applications.
Through Esri's 'Community Maps Program'- An integral part of the Living Atlas Community, Loudoun County's base map data is broadly available to the users worldwide.