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Impact Assessment
Impact assessment is the assessment of consequences (positive and negative) and identifying, predicting, evaluating the effects of a plan, policy, program, or actual projects prior to the decision to move forward with the proposed action. 
The types of impact assessments include global assessments which are the assessment of the impact of a project on an international level, policy impact assessment that assesses the impact of the change of a policy or the application of a new one and environmental assessment. 
Environmental impact assessment evolved during the 1960s when the public started growing interest in protecting the environment from the consequences of development. The aim was to give the environment a bigger part in decision-making and development planning. The success of the impact assessment as a planning tool led to its wider application, including applications to heritage. It was first applied to archaeological resources of known importance that were threatened with destruction due to infrastructure development projects. Impact assessment applied to archaeological and other cultural heritage resources is now gaining acceptance as an important development planning and heritage management tool.

Heritage Impact Assessment
Heritage is features belonging to the culture of a particular society, such as traditions, language, or buildings, that were created in the past and still have historical importance. Heritage impact assessment is a systematic process of identifying the probable results of a proposed policy or action on the cultural heritage of a place and its communities. 
In recent years the UNESCO World Heritage Committee examined the threats to World Heritage Properties and established that excessive tourism , demolitions or large-scale development such as roads, bridges, big buildings or tall buildings  threaten the identity of the World Heritage properties and their Outstanding Universal Value and even threaten sky line of cities and the key characters of their culture.
The Outstanding Universal Value is a tool consisting of ten criteria that help decide if a property should be put on the World Heritage List. The ten criteria are:

Representing masterpiece of human creative genius.
Important interchange of human values, on developments in architecture or technology, monumental arts, town-planning or landscape design
Contain superlative natural phenomena or areas of exceptional natural beauty and aesthetic importance.
An outstanding example of a type of building, architectural or technological ensemble or landscape which illustrates (a) significant stage(s) in human history.
To bear a unique or at least exceptional testimony to a cultural tradition or to a civilization which is living or which has disappeared.
To be an outstanding example of a traditional human settlement, land use, or sea-use which is representative of a culture (or cultures), or h man interaction with the environment especially when it has become vulnerable under the impact of irreversible change.
To be directly or tangibly associated with events or living traditions, with ideas, or with beliefs, with artistic and literary works of outstanding universal significance. (The Committee considers that this criterion should preferably be used in conjunction with other criteria).
To be outstanding examples representing major stages of earth’s history, including the record of life, significant on-going geological processes in the development of landforms, or significant geomorphic or physiographic features.
To be outstanding examples representing significant on-going ecological and biological processes in the evolution and development of terrestrial, fresh water, coastal and marine ecosystems and communities of plants and animals.
To contain the most important and significant natural habitats for in-situ conservation of biological diversity, including those containing threatened species of outstanding universal value from the point of view of science or conservation.

Importance of Heritage Impact Assessment
Heritage impact assessment provides the methodology to safeguard heritage resources against threats from development, or other scenarios of change, negotiates a balance between change, progress and conservation in ways that maintain the integrity of the threatened heritage and to mitigate the adverse impacts of development and change, enhancing and adding value to the heritage as a result.
It also ensures the continued relevance of culture in the community and protects the significance, meaning and function of heritage resources in the life of the community against exploitation, misuse and degradation as a result of change.
Another importance of heritage impact assessment is that it protects agains excessive tourism and proposed reuse plans heritage places.
Lastly, Heritage Impact assessment does not only assess threats from external or new proposed plans but also the threats from heritage conservation projects that often more damaging to the property than positively impacting it.

When to do an HIA
In most first world countries the assessment of potential impacts on culture and heritage is a requirement when performing an environmental impact assessment. However, there are many parts of the world where environmental impact assessment does not include consideration of heritage at all and HIA occurs in different contexts. For example when a funding agency requires it or at the request of the UNESCO World Heritage Centre or as a result of an NGO initiative in response to a proposed development or redevelopment plan.
The role played by community varies widely in heritage impact assessment approaches. In some cases an impact assessment is started only to seek the communities’ interest and support and in the others no assessment is considered unless it has total support of the communities. It is considered that the best mitigation plans are the ones proposed by the community. Clearly, the ideal lies somewhere between, highlighting the role of heritage impact assessment as a tool for negotiation and settlement.

Heritage resources to be assessed
There are different definitions of what Cultural Heritage should be assessed in a heritage impact assessment. Tangible, immoveable heritage is the main focus especially nationally and internationally protected heritage. Intangible heritage, however, is not considered in the heritage impact assessment processes.
Some heritage impact assessment guides recommend that given limitations of time and money, assessment should focus on the most significant properties in order of priority but the problem with this approach is that extra manpower is needed where someone has to identify what are theses most significant properties before the impact assessment itself is started. The other problem is that eventually only high-profile and legally protected resources are assessed. Resources which have legal protection or international recognition receive attention, while the undocumented archaeological sites and heritage potentials are ignored. Most guides provide no system on how to find the hidden heritage resources and instead rely on chance finds.

Important players and their roles
Because heritage impact assessment is part of larger development plans and development projects professional input is required at every stage of the process, including the early stages of screening to avoid over-reliance on existing heritage lists and inventories. There are many players at many different levels. There are those who decide whether or not a proposed project needs a HIA, those who write the scope and submit documents, specialists who actually implement the HIA, reviewers who assess the HIA and approve the project, and finally, those who implement and then monitor mitigation measures.
It is agreed upon that professional input is essential in the heritage impact assessment process. Baseline studies, including additional surveys and studies, as well as the assessment and mitigation design should be carried out by qualified and experienced heritage impact assessment professionals but approaches may vary depending on which level the professional input is needed or on the size and importance of the project or of the heritage resource itself. Some apply strict rules and requirements of professional qualifications while others do not employ professionals throughout the whole process at all. It is accepted that in the majority of cases it be carried out by someone who is appointed to be the heritage generalist. They will come from some other background, and will most likely be covering heritage on a part-time basis. Cultural and heritage professionals are consulted but not integrated into the actual process of identifying the heritage resource, the heritage impact assessment process itself or the implementation and the mitigation plans. As a result, there is a risk that poorly informed decisions at the early stages of the heritage impact assessment will negate later findings and compromise the effectiveness of the whole process.
-It is critical that HIA practitioners use an agreed upon and shared vocabulary to describe basic terms and concepts. Terms should be clearly defined at the beginning of every heritage impact assessment report. An assessment based on vague and inconsistent terminology has no integrity and can neither achieve its purpose nor help anrich the knowledge of heritage impact assessment.