More often than none the digital maps available for your Water Resources project may not comply with all the requirements needed for the correct application of the Arc Hydro Data Model. That is, there are quality issues that you must resolved before going any further. There are many reasons for this inconsistencies, one of them is the merging of information from diverse sources and the consequent different levels of resolution.

To do this you probably must employ all the GIS resources and abilities you have accrue after years of GIS struggle. Also, there are specifically customized tools to help on this refining tasks. As an example, below you will find a list of common inconsistent situations that you may encounter when trying to put your project together.

River Network without centerlines or discontinuous. One of the merits of having an Arc Hydro Model is the capability to build geometric networks. For water resources the river network is the case. Unfortunately, some of the available drainage networks lack of a continuous connection due to gaps at water body locations or some other places. For the Arc Hydro model you need to have a completely connected network so center lines need to be added following sound criteria. The thalweg criteria might be used for this purpose. Some agencies have developed software to tackle this issue and its references will be added as they become available.

Spurious Areas/Catchments. Sometimes polygon features that are supposed to be one get split during generation (in particular from Terrain Analysis generation). The typical case is the Catchments obtained as derivatives of a Digital Elevation Model (DEM). Due to quality issues the generated areas become inconsistent due to non-realistic subdivisions of the areas. Normally this can be easily corrected by using a common field that allows dissolving the features and putting them back together.

Ambiguous Loops. Sometimes we find complete physical connectivity that still is not logical. The flow direction cannot clearly be defined because there are ambiguous ramifications of the flow or because there are unresolved loops.

Disconnected Upstream Segments. Some upstream areas of your river network may have segments that are not connected to the rest of the network and you still need to incorporate them into the main drainage system. To correct this you may need additional information to make the complementation. A drainage line derived from a DEM may give support or additional maps at different scale can provide assistance.

Gaps. We normally take for granted that our data is or must be seamless. Unfortunately that is not always the case. Many polygon features have gaps or discontinuities that can introduce imprecision and voids in your project. Make sure your data is seamless in particular when you gathered data from different sources and with different spatial resolutions.

Duplicity. Again, if your data is coming from different sources you may have two spatial representations of the same feature. Make sure this is corrected by taking the ones with the greatest precision and reliability.

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