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significant new capability is the storage of features in the geometric network data model defined by ESRI. <br />This data model is discussed in the following sections. <br />2.4 NHD Data Model <br />To develop a link between NHD and HydroBase, it is important to understand the NHD design. The <br />USGS is making NHDinGEO available before completing the documentation for the data format. The <br />documentation has not been released, but information at http://nhd.usgs Gov/geodatabase_review.html <br />provides an overview of the data model. <br />The NHDinGEO contains several elements, including two feature datasets and several tables and <br />relationship classes. The main element is the Hydrography feature dataset, which contains the point, line, <br />and area feature classes as well as the network objects (Figure 2.2). Watersheds are the other feature <br />dataset, compatible with hydrography. The following Figure 2.2 is a diagram of the Hydrography data <br />model. <br />~ ESRd Ciasses:Peature ~ <br />kYdrpgraPfiY <br />=2 <br />NydrgAofrer <br />li asZ = <br />btypeFieldn -FType . esriFieldTypelnleger = 458 <br />Ada : asriFleldTypalnteger = 45flpC1 <br />Has2 =True} <br />u SubtypeFieldu -FType : esdFiefdTypednteyer = 460 <br />-I'Code : esril ieldTvoelnteaer = 4GWG <br />•ReachCode esriFieldType5lring <br />a5ubtypeField~ -F-type : esn Field'fypelnleger = 390 <br />-FCode : esriFieldAypelntager = 39004 <br />Tha prnmary distinction fi7r area [eature classes, NHpWaterbady <br />and NHDArea, is that She features in NHDWaterhody carry a ReachCrxie <br />attribute iqr linking assnraatetl data. <br />HydraLfne ffydroArea <br />NHDLine <br />{GeometryType = esriGeometryPdyline. <br />Hash =False, <br />Has2 =True} <br />esdFieldTypelr.teger = 411tl{} <br />{GeometryType = esriGeometry Polyline. <br />Hash =True. <br />IiasZ =True} <br />hlydroFlgwDirecligns = 0 <br />:oml^ : esri FieldTypglnte <br />The pritttary tlsstinctions ttetween line feature classes, NHDFlgwline <br />and NHDLirve are that Fhe NHDFlowline features can have a ReachCode <br />and M values For linking associated data, padirvpate in the 61ow network <br />participato ir. vertical relationships, and for artifical paths, may have a link <br />€o an associated wateabody feature ti~rough the W6 CgmID. <br />ho distinction is made yin po3n1 features with regard Fc uTich feature <br />Ivprs Car. carry a ReachCode to link associated data. <br />Figure 2.2. Hydrography Data Model in NHDinGEO <br />The object oriented data model uses classes to define objects. Some classes are in the ESRI core data <br />model. The triangles in Figure 2.2 indicate inheritance from a base class. For example, the top of the <br />diagram indicates that all feature (shape) classes are derived from the ESRI feature class. In NHD, <br />Page 4 of 17 ~fifversFde TeChnulo~yv fITC. <br />4s':rkv Resuurce.s Ergrnee: mg end ~C9 nsuNrng <br />