4.1. General evaluation method
In AHP, with a system as the research object, the influence of each factor in each level on the results is quantified. The evaluation method is characterized by clear logic, simplicity, and practicality and can be used for the systematic evaluation of unstructured characteristics and multi-objective, multi-criteria, and multi-period systematic evaluation. However, the evaluation method requires a large quantity of statistical data and faces the difficulty in determining the weight. In addition, the exact calculation methods of eigenvalues and eigenvectors required in AHP are complicated.
In PCA, original complex variables are transformed into several comprehensive variables for analysis. Factor analysis requires sample size and does not fully consider the relative importance of the indices themselves.
As traditional evaluation methods, both AHP and PCA have the advantages of clear logic, simplicity, and practicality, and are suitable for evaluating the systems with simple structures and clear hierarchical relationships. As a composite ecosystem composed of water and green spaces, the complex blue-green space may lead to its higher ecological carrying capacity and stronger vitality. Therefore, a hierarchical structure model can be established with AHP and the ecological carrying capacity of blue-green space can be comprehensively evaluated from two aspects of ecological support and environmental pressure (Zhao et al. , 2020). The main factors affecting the vitality of blue-green spaces can be analyzed with PCA. Although general evaluation methods have a clear hierarchy, the weight determination process is subjective and requires excessive calculation, thus affecting the accuracy of evaluation results. Therefore, in the future, the basic evaluation methods can be combined to improve evaluation results.