Precision Mapping

The United States GPS system was the first GNSS system available to consumers. Recently, many additional GNSS systems have gone live globally (GLONASS, QZSS, Beidou, Galileo, etc.), and many devices use multiple GNSS systems at the same time to improve coverage and accuracy.

GPS Accuracy: A well-designed GPS receiver can achieve a horizontal accuracy of 3 meters or better and vertical accuracy of 5 meters or better 95% of the time. Augmented GPS systems can provide sub-meter accuracy.

GPS Differential Correction: GPS receivers improve accuracy using two receivers because ground-based receivers can take accurate measurements of the error. As long as the stationary GPS receiver detects the same satellite signals as your GPS receiver, it can send you correction data based on its precisely surveyed location.

This augmented system broadcasts the corrected error in real-time along with the GPS signal. source

Read more on this topic and how it relates to Tallahassee - Leon County GIS by reviewing the information below.

Mobile

Bike Tallahassee

A cycling centric web site that is reliant of GIS for displaying popular cycling routes, scenic places, and resources to other outdoor activities.

Damage Assessment

In cooperation with Leon County Emergency Management, TLCGIS leads the technology and data collection portion of Tallahassee and Leon County's post disaster initial damage assessment efforts. TLCGIS also works with FEMA and SERT as necessary to certify Tallahassee and Leon County's preliminary damage assessments. Additionally, TLCGIS provides technology assistance to FEMA's Disaster Survivor Assistance program.

Products

TLCGIS leverages the vast mobile products available from ESRI. They include Collector, ArcGIS Explorer, Field Maps, Survey 123, etc. We often pair the applications with high precision GPS as required by the project.

Imagery

4-Band Orthoimagery

Metric quality orthoimagery is nominally collected every 3 years. The latest published imagery was captured in 2018 using a Leica ADS 100 single line scanner. The 4-Band imagery (blue, green, red and near-infrared) is published as an RGB mosaic and a false-color infrared mosaic. The spatial resolution is 0.5 feet. By using a network of independent checkpoints, The radial horizontal accuracy was tested to be 0.28 ft (RMSEr), Using NSSDA methodology, the horizontal accuracy at the 95% confidence level (called ACCURACYr and computed by the formula RMSEr * 1.7308) is +- 0.48 ft.

Oblique Imagery

EagleView 4-way oblique imagery is nominally collected for the populated portions of Leon County every 3 years. The latest data is from 2017 It has a spatial resolution of 6 inches in the urbanized area and 9 inches in the rural area. An update is scheduled for November, 2019 with a spatial resolution of 3 and 6 inches. This imagery allows for accurate measurements of ground objects but is not published to a standard.

Topographic Data

About

All topographic data products were collected and/or derived from a high-precision Lidar survey. All data products conform to guidelines and standards specified in the Florida County Option Orthoimagery Program Standards, 2018 USGS Lidar Base Specification 11-B4 , ASPRS Positional Accuracy Standards for Geospatial Data , and Florida Statues, Chapter 472 .

Breaklines

Breakline data was collected for use in producing digital elevation models (DEMs) and for hydrologic and hydraulic modeling. Hydrographic breaklines were collected to support hydrography-related requirements in the DEM. Single-line hydro breaklines were collected to enforce monotonic drainage. Area-based breaklies were collected for hydro-flattening the DEM for rivers and streams wider than 10 feet and for lakes and ponds greater than 0.25 acres. Hard and soft breaklines, along with bridge and saddle breaklines were collected for ensuring proper surface morphology. The breakline z-values were derived by point cloud interpolation.

Classified Lidar Point Cloud

A Lidar survey is collected countywide every three years. The latest was captured in 2018. The Lidar was collected to meet or exceed Quality Level 0 as specified in USGS Lidar Base Specification 11-B4 and ASPRS Positional Accuracy Standards for Geospatial Data. This requires that the data meet or exceed a non-vegetated vertical accuracy of 0.05 meters or 0.16 feet RMSEz and a minimum ground point density of 8 points per square meter . By using independent checkpoints, the vertical accuracy of the 2018 Lidar data was tested to meet 0.12 feet RMSEz in non-vegetated areas. The Lidar point cloud data was delivered in LAS 1.4 format and classified into the following classes: ground, low vegetation, medium vegetation, high vegetation, buildings, and water.

Contours

TA countywide set of 2-ft contours was produced from the DTM. The vertical accuracy of the DTM supports the create of half-foot contours as needed.

Digital Elevation Model

The Lidar ground points along with breakline data were used to produce a digital elevation model (DEM) with a spatial resolution of 2.5 feet. A hillshade model was derived from the DEM to facilitate shaded-relief mapping.

Digital Terrain Model

The Lidar ground points, breaklines, and the project boundary (Leon County) were used to create a triangulated digital terrain model surface (DTM). The DTM is stored in an ESRI LAS Dataset.

Feature Extraction

Classified LiDAR was used to create a Canopy Density raster, which represents the ratio of vegetation to ground as seen from above. A 5-ft resolution forest cover raster was generated from the Lidar point cloud using classes 5 and 6 medium and high vegetation respectively).

Spot Elevations

Spot elevations were collected at high and low points in the landscape and at road intersections.

Planimetric Data

About

All of the planimetric data except for the hydrography is updated to reflect changes over a 3-year periods. All data products conform to guidelines and standards specified in the Florida County Option Orthoimagery Program Standards, 2018 , USGS Lidar Base Specification 11-B4 , ASPRS Positional Accuracy Standards for Geospatial Data , and Florida Statues, Chapter 472 .

Bridges

Building roof outlines were compiled from aerial imagery for all visible buildings larger than 100 square feet. The buildings were extracted from the impervious surfaces data and published as a separate feature class.

Building Footprints

Building roof outlines were compiled from aerial imagery for all visible buildings larger than 100 square feet. The buildings were extracted from the impervious surfaces data and published as a separate feature class.

Hydrography

Includes rivers, streams, lakes, and ponds. These features are derived from the hydrographic breaklines compiled for the DTM.

Impervious Surfaces

The impervious surfaces data includes roads, bridges, buildings, driveways, parking lots, paved islands, sidewalks and tennis courts.