Spatial Data Infrastructure
Spatial data infrastructure encompasses the resources, systems, network linkages, standards and institutional issues involved from many different sources in delivering geo-spatially related information to the widest possible group of potential users [13]. Also, adopting this definition with general definition of infrastructures, it could be concluded that SDI has following characteristics [7,29]:

• Is a set of base capabilities;
• Is a general comprehensive system;
• Has multiple effective aspects in ICT utilization;
• Includes some stable and dynamic physical components;
• Provides important, fundamental and irreplaceable services.

An infrastructure is a kind of organization, which is the main basis for other organizing activity developments contributes necessary and different activities in sustainable development. In this process new and adoptive nature is created for each activity, preserving their usual specification through strategies and policies injection and integration assuring their effectiveness [6,17].

Reviewing the evolution of information societies and their organizing activities, ICT is being affected considerably in such a process as one of the dynamic and innovational components of SDI and a sustainable development enabler [6,16]. Despite the lack of proper ICT basis for supporting SDI primary activities, simultaneous and considerable evolution of SDI and ICT is defined in modern activities that results in ICT conversion to an infrastructure.

SDI development, which is treated as an innovation diffusion process [24,28] illustrates the evolution encountered in ICT infrastructure. Regarding this and the pervasive and valuable experience resulted from various successful SDI activities, SDI development could lead them to an applicable solution for ICT evolution in societies and moving towards sustainable development [27].

SDI Evolution
Tracing SDI trend, after one decade of its emergence (1990s), its concepts have been fulfilled considerably [4,5,26]. Balancing the GI generation and utilization concentration is the main evolution resulted new SDI definitions as follow [23,26]:

• Product-based model, which represents one of the main aims of an SDI development initiative, can be used to link existing and upcoming databases of the respective political/administrative levels of the community (Figure 1).
• Process-based model, which presents one of the other main aims of an SDI development initiative, defining a framework to facilitate the management of information assets (Figure 2).

These models are emerged from SDI concept capabilities in being breaking apart into their constituents (specialization), be optimized partially and finally hybridized as a unique fully optimized definition and model [5,26]. Important specification and optimization created in these models are as follow [26]:

• Definition of a suitable role for people as main factor which provides SDI with the required dynamic nature.
• Definition of social system as a set of related units used toward SDI aims realization resolves the required participation of all related stakeholders in society [27]. Social capability is defined in this system as the levels of general education and technical competence; the commercial, industrial and financial institutions that bear on their abilities to finance and operate modern, large-scale business; and the political and social characteristics that influence the risks, the incentives and the personal rewards of economic activity, including those rewards in social esteem that go beyound money and wealth. Social capabilities complement technological capabilities and combine in many different ways to generate economic growth [17]. Then, ICT will play an important role in building the social capabilities through information and generation and knowledge appliance for sustainable development.
• Clarification, simplification and optimization of SDI components and their relationships, which improves the SDI comprehension, modeling, evaluation and development [26,27].
• These components (like communication channels, awareness, knowledge infrastructure, persuasion, decision-makings, participation, utilization and capacity building) represent high level of adoption with ICT defined concepts and requirements [26,27].

Figure 1. Product-Base Model [26]



Figure 2. Process-Base Model [26]
Evolved benefits from these new definitions have created a general trend towards ICT infrastructures development and its effective contribution (esp. in European countries and Australia) [21,26].

Also, a hierarchical structure is defined for SDI, which comes from especial nature of geo-spatial data as they depend on scale and application. Regarding this structure, SDI treats in a pyramid, which its base (corporate level) is very similar to current foundation used for desktop GIS applications development and by its promotion to the peak, SDI converts to a global infrastructure (Figure 3).

Considering this structure, the following points could be illustrated [22,23]:

• From each level we could navigate to its upper and lower levels (Janus-Effect Specification)
• Each lower level provides the building blocks of upper levels (Part-Whole Specification);
• Each upper level transfers its overall definitions and backgrounds (strategy, policy) to lower level;
• The levels relationship fastens as they come closer (Near-Decomposability Specification);

These specifications and tangibility of this structure for societies, has clarified and simplified SDI concepts and developments more.

Besides that, SDI development, very similar to past experience of LIS development, utilized an organizational structure depicted as a pyramid defines public as its base and high level managers and decision-makers as its peak.

Integrating these structures with SDI models is presented in Figure 3.

Figure 3. Triple-tiered of SDI Hierarchy, Organizational Structure and Development Models [26]
The relationships presented above could result in a huge increase in SDI universality as it enjoys the following advantages [5,26]:

• Uses current activities of societies at each level and results in the least efforts duplication;
• Offers societies with different local situations, a wide range of opportunities in dealing with SDI development at all levels, ensuring their activities integration under the SDI concepts utilization;
• Represents national level of SDI as the most important level, which could foster the completion of other levels effectively.