Overlay

Overlay is, for most people, one of the most definitive operations of geographic information systems (GIS). Overlay makes it possible to combine vector and/or raster data for any areas based on a common coordinate system. The analytical capabilities of overlay make a critical contribution to geographic analysis in many fields.

However, overlay is a late arrival to geographic techniques and methods. The history of overlay prior to the late 1960s remains veiled in ambiguity. The technical and conceptual basis finds a parallel in the development of offset printing introduced during the early 20th century. The first published reference to the technique is from a 1913 landscape architect's plan for Billerica, Massachusetts. The cost and complexity of conducting overlays by hand led to geographers using representative fractions to numerically indicate relationships. For the analytical integration of soil types and crops in Montfort, Wisconsin, the possibility of overlaying transparent maps of each property was discussed, but until GIS became widely available, the expense and cartographic complexity dissuaded the pursuit of overlay as a technique for geographic analysis.

Ian McHarg's presentation of overlay, published in Design With Nature in 1969, involves the use of overlay to superimpose transparent thematic maps drawn in darker colors as the value associated with the property changes over an area. Overlaying layers of natural features, social features, and engineering considerations for transportation planning produces a composite in which darker colors indicate higher values and more conflicts for planning. Lighter areas in the composite map show areas with lower values and fewer conflicts.

The concept of overlay remains complex. The colocation of two properties is invaluable for geographic analysis, but the significance of colocation is subject to many considerations. The ingenuity of McHarg's use of overlay masks problems with ensuring that measurements of properties can be meaningfully overlaid, which logical operations reflect geographic relationships, and reoccurring questions about how properties are valued. The accuracy of overlay is a constant concern.

Overlay techniques have been greatly advanced since McHarg's seminal work and are combined with logic selection operations to process geographic information. Raster overlay has become analytically richer, and vector overlay has been integrated into database systems. Overlay techniques remain an important interface metaphor operation for integration operations that now are processed entirely by database software with limited geometric intersection processing. For all of the advances in implementation, overlay still requires a substantial amount of interpretation. Geostatistical techniques have become important complements and alternatives to overlay.