CalTopo Overlay Descriptions

This article and much of our Knowledge Base now lives at the CalTopo training site. This specific page can be found here. Please go there for the most up to date information, as well as more comprehensive instructions for using CalTopo.

In this article you will find descriptions of the overlays offered in CalTopo. Overlays go on top of base layers and provide additional data such as slope angle shading, motor vehicle use and realtime weather. Overlays are available in the drop down layer menu on the right hand side of the Map Viewer. Multiple overlays can be selected and shown at the same time on a single map.

The description of overlays below are grouped into the same categories as they appear in the dropdown layer menu. Scroll through or click below to jump to a particular category:

Map Overlays

Overlay Description
                                                                                                                                Based on USGS data sets. This overlay is the traditional way to visualize slope angle and changes in elevation on a topographic map. You can change the spacing of contour lines in the dropdown menu. Please note that the color is not editable; however Pro, Desktop and Team accounts can create a custom MapBuilder layer and select custom colors for contours.
MapBuilder Overlay
Displays roads, trails, hydrology and other features from the MapBuilder base layer. This allows you to overlay these features on any base layer that might not have this kind of data, such as aerial imagery.
Slope Angle Shading
A purely visual overlay that uses a color scheme for depicting slope angle. The color scheme can be set to fixed or gradient using the dropdown menu. Much of the United States now incorporates high-resolution LIDAR data from the USGS's 3DEP program. This provides significantly higher resolution, even allowing you to identify trails from relief shading. An interactive map showing high resolution coverage can be found here: High resolution offline downloads are larger in size than previous elevation data so be aware of that when downloading for offline use.
Public Lands                                                                                             Formerly named "Land Management", this overlay has been renamed to clarify the coverage. It identifies the land management agency (USFS, NPS, BLM etc) for a given area using a color scheme.
Parcel Data
Contains property boundaries and addresses for the majority of the United States, as well as ownership information for many of those properties. Due to the cost of licensing data, this overlay is restricted to Pro, Desktop and Team Accounts. Parcel data is licensed from, which is still growing and does not have ownership information for all counties yet. An interactive map showing coverage for ownership can be found here: Red indicates that we have the parcel boundary and site address only, blue indicates ownership information is available as well. This overlay will soon be available to download for offline use on the mobile app!
Motor Vehicle MVUM
Shows motor vehicle route designations and types. Clicking on MVUM Info will open up a motor vehicle use map key. This overlay incorporates data from multiple sources, including USFS and BLM. It is updated once yearly from all incorporated sources to the extent that new data is available; however we cannot guarantee data accuracy or that road conditions will be exactly as detailed in this layer. Recreate safely!
Fire History
Shows national coverage of wildfire activity since 2000. Fires are color-coded by year along a yellow-red spectrum. The year and incident name for all fires is shown, although you may need to zoom in for smaller fires. Please be aware that it can be hard to distinguish which label belongs to which fire in some areas with overlapping fire perimeters.
Fire Activity
Data comes from MODIS satellites (Aqua and Terra) and VIIRS which capture infrared imagery that is processed and used for hotspot detection. Generally hotspot data comes from a series of midday and overnight passes, capturing fire behavior at two points in the diurnal cycle. Each dot represents a hotspot detection and color indicates the age of the fire. Clicking on Fire Info will bring up a color key for fire age. If you zoom in on a hot spot, the semi-transparent circles represent the error radius. Each hotspot is labeled with the date and time of detection as well as the satellite. Hotspot detection can be filtered by satellite or shown with smoke coverage by using the dropdown menu. Smoke coverage is indicated as cross-hatched black polygons. More information on this overlay can be found at
GOES 17 Live Fire Detection
This is an experimental feature. Use caution when interpreting it. Data comes from the GOES 17 satellite, which covers the western US and provides hotspot detection on a near-real-time basis (5 minute intervals). The CalTopo map layer will auto-refresh to ensure you are always looking at the most current information and the time of the last satellite image is displayed on the layer in UTC. This can be really useful for tracking new or fast moving fires in between MODIS and VIIRS satellite passes. Be aware that GOES data has less spatial accuracy and is less likely to pick up small or low-intensity fires as compared with MODIS and VIIRS. It also does not reliably show you the boundaries of a given fire. More information on this overlay can be found at
Sun Exposure
Uses a blue-yellow-red spectrum where blue is completely shaded, red represents the maximum sunlight a slope can have on that day and yellow is 50%. Coloration is based on how directly the sun shines on a slope so glancing light counts less than direct full-on illumination. You can specify date and time or get an average across a day or month.

Weather Forecast

Overlay Description
Color Visualization                                                                                                                                 Combines many of the weather forecasting tools into one overlay. Use the dropdown menu to select between forecasted temperature, precipitation, snowfall, wind speed and wind gusts for the specified time frames. Data is based on the National Weather Service forecast grids and uses a color gradient for easy and quick visualization. Keys for the color gradient appear below the overlay and in the upper righthand corner of the map viewer.
Wind Plot                                                                                             Provides forecasted directions and speeds of the max wind speed or gusts for specific points in time, at 3 hour intervals up to 12 hours and then 6 hour intervals up to 36 hours. The same color chart as the Max Wind Speed overlay is used to indicate speed with short lines tracing direction (the thicker end of the short line indicates which direction the prevailing wind is blowing). The length of the lines has no meaning.
Forecast Grid
Interactive data layer showing the center points of the National Weather Service forecast grid squares. The center of each 2.5 km forecast grid is indicated by a dot that displays the 24 hr temperature low for the entire square. Mousing over a grid point shows the elevation used for the temperature interpolation. Clicking on the dot itself will open up a new tab with the NWS hourly weather chart. Different forecasts can be displayed using the dropdown menu.

Your Data

Overlay Description
Your Maps                                                                                                                                 Overlays map objects from your saved maps on the map you are currently working on. Saved map objects will appear as semi-transparent white overlays in the Map Viewer. Clicking on the overlay object will tell you which map it is and provide a link to the map itself. Clicking the link will open the saved map in a separate tab.
Your Tracks
Overlays mobile tracks that are saved to your account. Saved tracks will appear as semi-transparent overlays in the Map Viewer. Clicking on the overlay will tell you the name of the track and allow you to access the route profile and/or add it to your current map.

Shared Data

Overlay Description
Shared CalTopo Maps
Overlays map objects from publicly available CalTopo maps on the map you are currently working on. Similar to Your Maps, publicly available map objects will appear as semi-transparent white overlays in the Mapviewer. Clicking on the overlay will tell you which map it is and provide a link to the map itself. Clicking the link will open the publicly available map in a separate tab.
Shared Map Sheets                                                                                             Consists of a community resource of map sheets that improves support for local data that doesn't fit well with nationwide layers. The ability to view these items is available to all users, including free users. You can choose map sheets by use (summer, winter, MVUM etc) using the dropdown menu. Available map sheets are displayed in the Mapviewer over the areas they cover as white transparent boxes. Clicking on the box allows you to find out more info and add the selected map sheet to either your account or the map you are currently working on. Any map sheets you add to your map will appear in the left sidebar and under Map Sheets in the drop down layer menu. Any user can submit a map to be reviewed for this layer at This will soon be available to download for offline use on the mobile app!
MapBuilder Features
Turn on this overlay to get more information about features included in the MapBuilder layers such as roads, trails and peak names. Hovering over each feature will highlight them; clicking on them will tell you exactly what it is. Clicking on the name of the feature will open up a new tab with further information from an outside website on that particular feature
Mountain Project Areas
Displays the location and predominant climbing style of rock climbing areas that are on the Mountain Project website. Climbing areas are indicated by the Mountain Project icon and the color of the icon indicates the predominant style of climbing in the area: orange for sport climbing, red for trad climbing, purple for bouldering and blue for toprope. Black icons indicate areas with multiple climbing styles where no single style predominates. Clicking on the icon will display the name of the area and the number of each type of route. Clicking the name of the area will open the Mountain Project page for that area in a separate tab. Where available, approaches to climbing areas are indicated on the map as semi-transparent line overlays. Clicking on the line overlay will display the name of the approach and clicking on the name will open up the Mountain Project page for that approach in a separate tab. This is a trial partnership with Adventure Projects and is currently only live on the web.
MTB Project Trails
Displays the location and difficulty of mountain biking trails that are on the MTB Project website. The color of the trail indicates the general difficulty: green for easy, blue for intermediate and black for difficult. Clicking on the trail displays the trail's name, its difficulty and a photo of the trail. Clicking the name of the trail opens the MTB Project webpage for the trail in a separate tab. This is a trial partnership with Adventure Projects and is currently only live on the web.

Realtime Data

Overlay Description
SnoTel Sites
Displays hourly data from SNOTEL stations. Standard stations are shown in red and stations with wind sensors are shown in blue . Each station has a label that indicates snow depth in inches, current snow density and current temperature. Clicking on a station will bring up a dialog with the 7-day sensor history and a link to the station's official webpage, where you can check out all kinds of additional charts and tables.
Water Gauges                                                                                             Displays realtime data from USGS water gauges. This overlay includes 7,500 stream and reservoir gauges. Each stream gauge has a label that indicates flow in cubic feet per second (cfs) and water temperature (where available). Reservoirs show the current capacity in acre feet (acft). Clicking on a station brings up a graph of the last week and a link to the station's official webpage
Weather Stations
Displays NOAA weather stations. Data is screen-scraped off of a web page rather than coming through a documented web service so it may be less reliable than the other realtime overlays. Each station has a label that indicates temperature and wind speed. Stations with wind speeds 10 mph and under will be shown as a black dot and stations with wind speeds over 10 mph will be shown as a black arrow indicating wind direction. Clicking on a station brings up additional details and a link to the station's official observation page.

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