Overlay Descriptions

Map Overlays




The contours overlay is based on DEM, or Digital Elevation Model, data sets. This overlay will help you add topographic information to an otherwise “flat” map such as aerial imagery or a street map. Contours are offered at Mixed, 40 ft, 10 ft, 20 m, and 5 m intervals.

You can choose change the spacing of contour lines in the dropdown menu. For this overlay, the color of the lines is not editable; however Pro, Desktop, and Team accounts can create a custom contours overlay using the custom MapBuilder layer function, and choose the color there.

This overlay is available for download for offline use in the mobile app.

MapBuilder Topo

Described in detail at the start of this chapter, the MapBuilder overlay displays roads, trails, hydrology and other features from the MapBuilder base layer. This overlay will add context to aerial images or other base layers that do not call out roads and trails.

Slope Angle Shading

This purely visual overlay 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 area covering the United States now incorporates high-resolution LIDAR data from the USGS's 3DEP program. This provides significantly higher resolution than past data, 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.

Slope angle shading should be used in conjunction with your backcountry travel and terrain assessment tools. Do not rely on it for absolute truth.


New as of November 2020, this layer shows the prominent geological forrmation type. As you zoom in, it will show the label for the type of substrate. This data is sourced from the USGS. You can find a similar map based on the same data with more details here.

The layer comes with this key, which you can also open inside the map viewer by clicking the link below the map overlay checkbox.

Public Lands

The public lands overlay identifies the United States land management agency (USFS, NPS, BLM, State, local, or private) for a given area using a color scheme.

You are responsible for determining the regulations for travel and use when entering lands managed by each particular owner or manager.

Formerly named "Land Management", this overlay has been renamed to clarify the coverage.

Parcel Data and Structures Overlays

This overlay is restricted to Pro and higher subscription levels. It is then also available for offline use in the app.

The Parcel Data overlay shows property boundaries and addresses for the majority of the United States, as well as ownership information for many of those properties. A dropdown menu allows you to choose if the layer displays ownership data, address number, or boundaries only.

Parcel data is licensed from LandGrid, 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 areas indicate that we have the parcel boundary and site address only, while blue areas indicate ownership information is available in addition to the boundary and address info.

Meanwhile, the Strucutres overlay depicts known structures as polygons at high zoom levels and as dots at low zoom levels.

Motor Vehicle Use Maps (MVUM)

The MVUM overlay shows motor vehicle route designations and types. 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.

MVUM in Overlay menu showing link to key.

Find the key by clicking the blue link for MVUM Info below the checkbox.


The MVUM legend.

Fire History

This layer depicts wildfire perimeters in the US since 2000. Fires are color-coded by year along a yellow-red spectrum. The year and incident name for all fires are 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

This data comes from MODIS and VIIRS satellites which capture infrared imagery that is then processed for hotspot detection. The satellites are not geostationary, so hotspot data comes from a series of midday and overnight passes, capturing fire behavior at two points in the diurnal cycle. Therefore, this overlay, while geographically more accurate than the GOES 17 overlay, is not meant for immediate life-safety decisions.

Each dot represents a hotspot detection and the radius of the circle reflects the uncertainty of the position of the hot spot. Color indicates the age of the fire. To see a color key for the layer, click on the Fire Info link just below the checkbox after you activate it. Each hotspot is labeled with the date and time of detection as well as the satellite source. Temperature indicates the heat relative to the ambient temperature.

Display Options:

  • VIIRS 375m: Show VIIRS detections only, plus perimeters.
  • MODIS: Show MODIS detections only, plus perimeters.
  • All Satellites: Display both VIIRS and MODIS detections, plus perimeters.
  • w/ Smoke: Adds a polygon of cross hatching to indicate dense smoke.
  • VIIRS Progression: Shows the changes and movement of the fire hot spots over time. Key is in the Fire Info link.
  • Perimeters Only: Won’t show hot spots, but only the fire perimeter.

Remote fire detection is a complex science and can be difficult to interpret, especially if you or your property are threatened by fire. We provide more information on this overlay and the data behind it in this blog post.

GOES 17 Live Fire Detection

This is an experimental feature. Use caution when interpreting it, as the location of hot spots can be off by more than a mile.

This data comes from the GOES 17 satellite: a geostationary satellite, which covers the western US and provides hotspot detection on a near-real-time basis at approximately 5 minute intervals. This overlay will auto-refresh to show the most current information. The time stamp of the last satellite image is displayed on the layer in UTC (the time zone cannot be changed at this time). 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 in this post.

Sun Exposure

The sun exposure overlay 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: glancing light counts less than direct full-on illumination. You can specify date and time or get an average across a day or month using the menus.

7:30 am Sun Exposure October

Mt. Rainier 7:30 am Sun Exposure in October.

8:00 am Sun Exposure October

Mt. Rainier 8:00 am Sun Exposure in October.

8:00am Sun Exposure July

Mt. Rainier 8:00 am Sun Exposure in July.

This overlay takes a significant amount of processing power and can slow your browser or cause it to hang, especially if zoomed out to cover a wide geographical area.

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Color Visualization

With this overlay you can view color-coded forecast representations across a wide area. It 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, and the other menu to set the time frame. 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

A user favorite, the wind plot displays arrows showing current or forecasted direction and average wind speed or gusts for specific points in time. View predictions at 3 hour intervals up to 12 hours and then 6 hour intervals up to 36 hours.

Color corresponds to speed. The same color chart as the Wind Speed color visualization 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 (think of a comet where the head of the comet is the direction of travel). The length of the lines has no meaning.

Forecast Grid

This is an 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 defaults to displaying the 24 hr temperature low for the entire square. Different forecasts, including temperatures, winds, and precipitation values, are available using the dropdown menu.

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.

Avalanche Forecast

In close partnership with the American Avalanche Association, the USFS National Avalanche Center, and Avalanche Canada, CalTopo now includes avalanche forecasting for the US and Canada.

These forecasts are delivered in real time from forecasting organizations and displayed on the Avalanche Forecast layer in both the app and on the web. We focused a great deal on ensuring accurate and up to date forecast information is shown. For obvious reasons this layer requires internet connectivity.

When you activate the overlay to the map viewer, it will fade, then disappear as you zoom in. This is intentional.

Click on any forecast area to pull up a box with the local message and forecast. Follow the link at the bottom of this box to the full forecast page for additional information.

We strongly encourage users to click on the forecast to get further forecast details and discussion. We also recommend that all backcountry travelers spend time educating themselves on winter travel. The American Avalanche Associate and Avalanche Canada both provide excellent educational resources for you to do so!

Jasper Avy forecast box nov 20, 2020

The forecast will always include the name of the area, the dates and times the forecast is valid, a status for the rating, and a link to the forecast page for more information. It may also include a short description, and relevant graphics as you see here.

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Your Data



Your Maps

This overlay shows objects from all of your saved maps over your current map. The saved map objects will appear as semi-transparent white overlays in the Map Viewer. Clicking on an overlay object will open a box to show the name of the object and the 4-digit ID of the map where the object is saved. Clicking the link will open the referenced saved map in a separate tab.

For example, you record a track and save it to Map A. You open Map B at the same location, and turn on this overlay. You will see the “ghost” track on this map and can click the link to open Map A.

Your Tracks

Tracks you record in the mobile app are saved to Your Tracks automatically. This overlay allows you to view all of those tracks over the current map. These saved tracks will appear as semi-transparent overlays in the Map Viewer. Clicking on the overlay will open a box with information about the track including the distance, date recorded, a link to view the track profile, and a link to add the track to the current map.

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Shared Data



Shared CalTopo Maps

This overlay acts similar to the overlay “Your Maps”, but it displays objects from all public maps saved in CalTopo. Also similar to Your Maps, publicly available map objects will appear as semi-transparent white overlays in the Mapviewer. Clicking on an overlay object 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

Map Sheets are a type of custom layer, usually a georeferenced PDF that is a local or regional map. This overlay functions as a community resource to gather and share local data that doesn't fit well with nationwide layers. The ability to view these items is available to all users, including free users.

Map sheets are sorted by use and you can choose a category (summer, winter, MVUM, etc) using the dropdown menu next to the overlay name. A selection of available map sheets are displayed in the Mapviewer as white transparent boxes over the geographical areas they cover.

Clicking on a box on the map allows you to find out more info and add the selected map sheet to either your account or to the current map view. When you add map sheets to your map, they will appear as an object in the left sidebar as well as a stacked map sheet in the layers area. Please see the Map Sheets section for more info about adding, stacking, and removing map sheets.

Any user can submit a map to be reviewed for this layer at https://caltopo.com/submitmap. 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.

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Real Time Data



SnoTel Sites

SNOTEL is a system of automated snow monitoring stations across the western US and Alaska. The overlay displays hourly data from these 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 Gagues

With this overlay, you can view real time data from USGS water gauges, including 7,500 stream and reservoir gauges across the US. 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

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.

Data is sourced from MesoWest.

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