Interdependence of GIS Layers

Oftentimes GIS data conversion efforts focus on converting a single layer into digital format without regard to the relationship one GIS layer may have to other GIS layers. For example, the scope of road mapping for enhanced 9-1-1 may be to obtain a physical representation of road networks, irrespective of the importance of roads to other GIS layers, such as cadastral, or the dependence of the road GIS layer on geodetic control or orthoimagery. Even seasoned GIS professionals can develop a degree of tunnel vision when focused on a specific project. Some data conversion projects may take years to complete, especially when the project is a collaborative effort that involves many federal, state, tribal and local government agencies.

The tendency to focus exclusively on a single layer usually manifests in a project's structure, funding, and allocation of human resources. The lack of a multi- layer approach to GIS conversion projects is ironic considering that the GIS paradigm fosters collaboration and partnering on projects (see "Learning from the GIS Paradigm," February 2001, pp. 38-40). It is important to recognize the interdependencies of various GIS layers in order to take a holistic approach to GIS layer development. In this way funding and resources can be leveraged, and the layers, thus developed, will properly integrate spatially.

The Federal Geographic Data Committee has promoted the I-Team Initiative that "addresses the institutional and financial barriers to development of a National Spatial Data Infrastructure (NSDI). An I-Team helps the community come together to produce, steward and exchange a portion of America's framework and other geographic data assets." The I-Team Initiative provides a structure for agencies from the federal, state, tribal and local governments, and the private sector to promote data development of the seven framework layers: Cadastral, Elevation, Geodetic Control, Governmental Units, Hydrography, Orthoimagery, and Transportation. Nearly every state in the U.S. has now adopted the I-Team structure as a way of coordinating GIS framework development.

Funding issues
One of the main issues confronting the I-Teams in the various states is how to secure funding for the collection and maintenance of the framework layers. Montana is one of those states that adopted the I-Team structure early on, and in fact, added five additional layers to the FGDC 7. When Montana brought its I-Team champions together to identify and discuss common funding issues, that team immediately recognized that a comprehensive approach to funding would likely yield more support than would a piecemeal, layer-by-layer approach. The consensus was that asking federal agencies or the state legislature, or any other potential funding sources to support GIS data conversion and maintenance would have greater chance for success if a total package could be presented rather than asking for support for one layer, then coming back in a few years to ask for support for another layer. The decision to take a comprehensive approach to funding was an important one. Although there are some layers that are in immediate need of funding, and taking a comprehensive approach will take time, a comprehensive approach should demonstrate to potential funding sources that the GIS community will work to maximize the benefit for the money invested.

Leveraging Efforts
The decision to take a holistic approach to funding also raised a couple of other important challenges: how to prioritize layers for funding, and determining which layers are most important in terms of supporting decision-making and supporting other layers. It is important to note that creating data layers (such as a cadastral layer) is not the end goal of GIS. Data are merely the raw materials for the GIS applications factory. Indeed, neither are maps the goal of GIS. Data are converted to digital form in order to support GIS applications, and maps are merely a way to represent results in a visual form. Although it is true that in some situations a primary goal may be merely to provide rapid access to spatial information, typically GIS data are created in order to be able to answer questions that we ask about our world. Moreover, because our world is increasingly complex, there are more questions being asked of the data, by increasingly more agencies, companies and individuals. Thus, the variety of GIS applications is staggering and constrained only by human imagination, which seems boundless. Therefore, the number of applications that demand GIS data is continuously increasing. That is why it is very important to recognize the interrelationships of the various GIS layers in order to leverage funding and efforts, and to promote data sharing, because all the applications depend on GIS data.

Analyzing the Layers
When the Montana I-Team Champions mapped out the relationships of the framework layers, some of those dependencies became apparent as shown in the chart in Figure 1. There are two ways to view this chart, both of which provide important information. The first way, shown in Figure 2, shows the foundation layers from which other layers are built. We can see from Figures 1 and 2 that Geodetic Control plays a critical role in GIS data conversion for nearly every framework layer. There are only two layers (Soils and Land Cover) that can be developed somewhat independently of Geodetic Control. In addition to Geodetic Control, Elevation Data and Orthoimagery are the other two most important foundation layers. There is no framework layer that can be built without using at least one of these three layers.

The other information that can be gleaned from the chart are the Dependent Layers, shown in Figure 3, which depend on other layers for their creation. For example, in order to create Orthoimagery, Geodetic Control and Elevation data must be obtained. We can see from Figure 1 that all framework layers are dependent upon other layers, with the exception of Geodetic Control. Geodetic Control is the only framework layer that can be built independently. It is interesting to note that the Infrastructure layer depends upon nearly every other layer for its creation. In the Montana context, Infrastructure is a generic term denoting many of the features built by man, and includes so-called "critical infrastructure" such as bridges, dams, power generating facilities, and the like, which are important to homeland security. One can see from the relationships in this chart that in order to develop the Infrastructure layer (critical or otherwise) many of the other layers must already be in place or developed first.

This information helps us to understand the importance of individual layers and the interdependence of the various framework themes. From this, we can prioritize layer development. For instance, for any layer to be created, Geodetic Control must exist. Also, the Cadastral layer is supported by Geodetic Control, Transportation, Orthoimagery and Hydrography. Any social, political, or economic issues that use GIS applications for decision-making, and thus demand one of the framework layers, will in most cases also require one or more other framework layers. Therefore, GIS data development projects should take a holistic approach that supports the development and use of the foundation layers in addition to the layer of interest.

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