Exploring Basemaps and Overlays




The NHD Viewer serves as not only a gateway for downloading NHD and other data themes from The National Map (TNM) but also provides several basemap options for observing NHD data. Overlays allow for individual layers to be turned on and off and add additional flexibility in what the User is viewing. In this tutorial, you will learn how to switch between the different basemaps in the NHD Viewer and turn overlays on and off. You will also learn that NHD layers become visible at varying scales.


Step 1 Open the NHD Viewer


Go to the NHD Viewer.


What is the NHD Viewer?


·        The NHD Viewer has four basemap options.


What is a basemap?


      The NHD basemap is the default for the NHD Viewer. You can turn the shaded relief on or off by clicking the check box beneath the basemap button. At the full extent of the NHD basemap, you will see a generalized flowline layer combined with Watershed Boundary Dataset (WBD) HUC 2 Region boundaries in red.


       What is a Hydrologic Unit Code?


      Other reference layers such as shaded relief, state boundaries, and densely populated cities help you become oriented. As you zoom in,  more hydrography layers become visible and Watershed Boundaries are further divided into smaller hydrologic units.




Step 2 Toggle between the different basemaps


     The Basemap options appear in the top right hand corner of the viewer.




      Click the button for "Base Map". This is The National Map (TNM) basemap and contains additional layers of information such as Roads, Forest Service and Indian Reservation Boundaries, and much more. Notice that the hydrography layer is more generalized than it is in the NHD Basemap. 


·        Click the button for "Imagery". Hydrography information shows above imagery. The white pixels are an issue that has to do with the hydrography information contrasted against the imagery. This issue will  improve in the future.


·        Click the button for  "Blank". This basemap allows for the viewing of overlay data by itself.


Step 3 Explore the Hydrography Overlay


Choose the Blank Basemap.



Notice that all hydrography layers are turned off, and you are viewing the outline of North America.

The Base Data layers are on the left side of the Viewer Window under the tab called overlays.


What is a overlay?



Turn on the Hydrography (NHD) Overlay by clicking the box next to its name. A status window will tell you that data is loading.


Zoom to a larger scale. Pan around the map and note that the data draws slower than it would with the NHD basemap. In general, the map performance will get slower as the scale gets larger.


·        Expand the hydrography layer. Note: the Overlays window can be minimized to increase map real estate by clicking the arrow on the Overlay window.



There are three group layers in the Hydrography Overlay:


1.  The Watershed Boundary Dataset (WBD) - This group contains all features and labels from the WBD.


2.   Medium Resolution - This group contains all medium resolution NHD features and labels. These layers are visible from scales 1:577,791 to 72,224.


3.    High Resolution - This group contains all high resolution NHD features and labels. These layers are visible from 1:72,224 and larger.


What is the difference between medium and high resolution NHD?


Expand the High Resolution group layer by clicking the box next to it.


When you hover your mouse over the layer name "High Resolution" a popup will tell you at what scale the group is visible. High Resolution is visible at 1:108,336 and larger.


When you expand the High Resolution group, you will see that it contains two groups. One is called "Labels", and the other is called "Features". Labels may be turned off so that features can be viewed independently.




Step 4 Exploring NHD Features in the Viewer


Expand the Features group. You will see the NHD Feature classes which can each be turned on or off based on your preference.


To better understand these feature classes see the NHD Feature Catalog.


There are two Flowline layers displayed in the Hydrography overlay; Flow Direction Arrows, and Flowline. Flow Direction Arrows provide flow direction for flowlines at large scales (1:36,112 and larger). Flowlines contain the line work for NHD features that have flow. These are separated into two layers so that you can turn the flow direction arrows off in case you do not want to view flow direction at large scales.  


Expand the Flowline- High layer. You will see all the feature types that make up the NHDFlowline feature class. The symbology, or color of the line associated with the feature, appears next to the feature type (FType). You can use this to understand which FType you are observing in the NHD Viewer.


Zoom to a scale of 1:36,112. The flow direction arrows are now turned on.


Expand the Flow Direction Arrows layer and note that each flowline feature type has an arrow. In addition to arrows, this scale also enables some stream river feature codes or FCodes to become distinguishable. You will see that Perennial, Intermittent, Ephemeral, and StreamRiver (no attributes) are symbolized differently.


Test your Skills


1)     In the search window type Youghiogheny River. Now using the steps in the tutorial please answer the question below.


2)   Find the Dam south of Deer Lake. Is which direction is the water flowing in this area?


      Answer Key