Synergizing smartphones’ onboard GPS capability with KML files

By Jay Satalich, P.L.S., GISP

At Caltrans District 7 in Los Angeles, we use the onboard GPS capability of smartphones to navigate in real time to the locations of proposed aerial targets and National Geodetic Survey (NGS) control stations.

Keyhole markup language (KML) files are created in the office using desktop GIS, then downloaded to smartphones for use in the field. We create KML files specifically for use by our surveyors during every aerial mapping project within Los Angeles and Ventura counties.

FIGURE 1. Highway Interchange displayed on a smartphone using Google Earth App for Android, (ground targets in blue, flight information for pilots in red and green). Airborne GPS positioning aids in controlling aerial photography as the pilot navigates from exposure to exposure. A flight management system automatically triggers the camera or sensor once it reaches the exposure station in the air.

KML is an extensive markup language (XML) notation for expressing geographic annotation and visualization within Internet-based, two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth — originally named Keyhole Earth Viewer.

The aerial target layer also shows the proposed locations of stereo model limits on the smartphone. A stereo model is the overlapping portion of two adjacent aerial images. Each typically has a 60 percent overlap with its adjacent image, so it can be viewed and mapped in stereo. The ground control is combined with the airborne GPS to provide the orientation of the individual exposures, and it establishes the coordinate space of that imagery for any subsequent products.

Having the stereo model limits as a data layer becomes a handy piece of information in the event an aerial target must be relocated because of unfavorable field conditions. The heads-up capabilities of GPS aboard the smartphones and KML files can also show the easiest path to reach either target location or control stations. The NGS control station layer hyperlinks to the NGS website, so the field surveyor always has the recovery note available in an electronic format.

The field surveyors are also given hardcopy maps of the target locations and control stations, but those are now only used as a backup to the KML files loaded onto the smartphones.

FIGURE 2. Phone Screen with station description from NGS database (above).

FIGURE 3. The user arrives here via a hyperlink from another screen (FIGURE 2).

We have found that leveraging the onboard GPS capability of smartphones with GIS-based data layers in the field has increased production. Using smartphones provides the surveyors with information more concisely and clearly. This information enables surveyors to make better decisions in the field.

One example is identifying inaccessible areas. If the field surveyor sees that an aerial target can be moved to a different location that provides easier access, it can save time and guesswork.

This information is also valuable in rugged areas because the field surveyor may need to identify the location of hiking trails or while surveying in the desert, or identify the location of aerial targets in areas that are either lightly inhabited or have few landmarks. The project surveyor can tailor datasets specifically to project needed by the field surveyors.

Once the aerial targets have been placed and the NGS control stations recovered, the field surveyors then position the aerial targets and control stations using carrier-phase GNSS. This gives us the centimeter-level accuracy needed to control the aerial photography during our mapping projects.