1. Flow accumulation: cumulative sum of water flowing across a surface. Flow accumulation model outputs are grid cell layers in which each cell has a number representing the total number of cells that drain through that cell累计流过表面的水量，Flow accumulation model的输出是网格单元层，每个单元格都有一个数字，表示流经该单元的水量
2. data type: spatial data, attribute data, topology
3. Topology is a collection of rules that coupled with a set of editing tools and techniques, enables the geodatabase to more accurately model geometric relationships
4. topology uses:
a) Data validation数据验收
b) Spatial analysis, dissolving the boundaries between adjacent polygons with the same attribute values使用相同的属性值溶解相邻多边形之间的边界
5. why topology is useful:
a) to provide a rigorous, automated method to clean up data entry errors and verify data
b) typical digitizing procedure: digitize all lines -> build topology -> label polygons -> clean up slivers, dangles, and under- and overshoots -> build topology again
(repeating the clean and build phases as many times as necessary)
6. A topological model represents spatial objects as an underlying graph of topological primitives. The primitiveswith their relationships to one another and to the features whose boundaries they represent, are defined by representing the feature geometries in a planar graph of topological elements将空间对象表示为topology基元的基础图。通过在topology元素的平面图中表示特征几何来定义基本体及其相互关系以及它们表示边界的特征
Two views:
a) Features: features are defined by their coordinates or features are represented as an ordered graph of their topological elements.
b) Topological elements
7. A layer of polygons can be described and used: As collections of geographic features (points, lines, and polygons) + As a graph of topological elements (nodes, edges, faces, and their relationships)
8. spatial objects: point, line, and area features; topological primitives—nodes, faces, and edges
9. Many vector datasets contain features that share geometry, when editing these layers, features that are coincident should be updated simultaneously so they continue to share geometry. Topology allows to perform edits in this manner许多矢量数据集包含共享几何图形的特征，在编辑这些图层时，重合的特征应同时更新，以便它们继续共享几何图形。拓扑允许以这种方式执行编辑
10. Topology rules define the permissible spatial relationships between features. The rules you define control the relationships between features within a feature class, between features in different feature classes, or between subtypes of features定义了特征之间允许出现的空间关系。自行定义的规则将控制同一类要素之间的关系、不同类中要素之间或要素子类型之间的关系
11. Spatial relationships：express specifically how features share coincident(重合) geometry along with the rules for the behavior of their spatial representations.