Screen shot example of citrus grove boundaries overlaying one-meter resolution aerial imagery.

The census is a major undertaking considering there are nearly 40,000 groves, totaling 700,000 acres, across peninsular Florida dedicated solely to citrus production. On average, about 130 trees are planted per acre, resulting in a total citrus tree count of around 91 million. The majority of harvested fruit is processed into orange or grapefruit juice. Citrus is estimated to be a $9 billion a year industry and, thus, economically important to the state of Florida and a major component of U.S. agriculture.

During the last year, NASS/FFO modernized its methodology for maintaining and conducting the biennial census. Previously, the citrus grove location information was stored using paper-based maps. The sheets, sized 36 by 36 inches and scaled roughly 1:8,000, were copied from annotated panchromatic aerial photographic transparencies via an ozalid process. The products contained enough imagery detail that mature tree stands could be delineated by a photointerpreter and located by a field enumerator.

On each map, known grove boundaries were penciled by hand. To define the current grove status, the previous census' maps were used as a basis for change assessment and compared against most recent aerial photography and field report information. To cover the entire citrus region, tens of thousands of groves were delineated, which required an inventory of more than 1,000 of the ozalid-style printouts to be maintained using manual cartographic methods. A large effort in terms of time and labor was constantly required for upkeep. Also, ancillary tables containing grove statistics, such as tree count, variety, plant date, and row spacing, had to be managed in parallel (albeit stored digitally).

Florida orange trees in spring flush.

An improved system would no longer require the repetitive redrawing of groves with each census, especially since the majority of groves do not change, and would allow groves to be physically linked to their attribute data. It would also give NASS more flexible and consistent map products along with a better means for photointerpreting change areas. ArcGIS Desktop (ArcInfo) was a natural tool for such a task and ready to implement because USDA has a department-wide site license for Esri products. Furthermore, ArcGIS is already utilized by NASS for a variety of thematic and analytical projects, so in-house consulting was available.

The most laborious step of the transition consisted of heads-up digitizing of year 2004 boundaries for all citrus groves across the state. To accurately define the grove boundaries within a GIS, high-quality digital orthorectified imagery was required for ground reference. Fortunately, a one-meter resolution dataset had just been flown in early 2004 during a statewide endeavor sponsored by Florida's five water management districts. The provided aerial data was easy to ingest and manage within ArcGIS using an image catalog. The imagery was of sufficient detail to allow grove boundaries to be comfortably delineated within three meters of actual boundaries. Many staff members, with a variety of backgrounds and skill sets, helped in digitizing the grove boundaries. Most had never used any type of GIS software previously but took to the task once they realized the benefits of converting the groves into digital format.

The personal geodatabase data model proved a good fit for warehousing the citrus data for several reasons. First, the geodatabase can maintain a one-to-many database record relationship (many groves are of mixed plantings and, thus, have more than one data record). Second, it allows for topology checks against one or more data layers (having overlapping groves is not desirable). Third, the geodatabase's design is concise, which allows feature classes and data tables to be organized and related within a single file (this eases management). And finally, the personal geodatabase does not require support of a database administrator (this results in less expensive infrastructure).

The building of the citrus GIS inventory was completed during spring 2006. Citrus enumerators still rely on paper maps but are now using improved ones created from ArcInfo software's ArcMap application, utilizing the grove layer overlaying current digital orthoimagery, the Public Land Survey System grid, and roads data. They are able to modify map layouts to suit cartographic needs and benefit from the increased detail the new maps afford compared to the former ozalid reproductions. Aerial photointerpretation and delineation of grove changes continue via visual inspection of before-and-after images (2005 imagery flown via the USDA's National Aerial Imagery Program is now being analyzed against the 2004 imagery) but are now performed digitally within ArcInfo.

Reference map for Hardee County showing distribution of citrus groves.

The biggest advantage of the citrus GIS, however, is the ability to more quickly document and publish rapid changes to the tree numbers. Citrus growers in Florida have had many negative impacts to their livelihood in recent years, including large losses in capacity from disease (especially citrus canker), storm damage (increased hurricane prevalence), and rising land values (groves becoming urbanized). NASS believes the GIS will help transition the census to an annual cycle allowing the Florida citrus industry to better monitor changes and, thus, improve on policy decision making and citrus production forecasting.

More Information

For more information, contact David M. Johnson, geographer, USDA NASS/Research and Development Division (tel.: 703-877-8000, ext. 169; e-mail: dave_johnson@nass.usda.gov), or Candice Erick, economic research associate, USDA NASS/Florida Field Office (tel.: 407-648-6018, e-mail: candi_erick@nass.usda.gov).