Data Downloads

Basic Data and Monument Sets Sample Animations

Sample Photos and Terrain Map Tile Data Information

---

Basic Data and Monument Sets

Downloads	Monument Data
Basic Data Set Zip File - Size: 16mb Basic Download Information PDF File - Size: 2mb	Monument data and information can be found on the Cobb County Cooperative GPS CORS website

<< Back to Top

---

Sample Animations

	Lost Mountain Area Windows Media Player - min. sec.
	The Historic Mableton Preservation and Improvement Plan Windows Media Player (6.3mb)

<< Back to Top

---

Tile Data Information

Base Map Data Set

Overview
Development of Cobb County’s digital orthophoto base map began in February 2000 with the taking of color aerial photography covering the entire county.  The photographs were used to produce highly accurate digital maps of roads, water features, and 2’ contours depicting the elevations of Cobb County’s terrain (topography).

Subsequent updates have been made annually since 2003, with full updates of the digital orthophoto base map, and partial updates of the Digital Terrain Model (DTM), 2’ contours, and limited planimetric mapping (transportation and hydrography).  In 2006, the aerial photography was acquired using digital cameras in an attempt to shorten processing time and improve quality.

The positional accuracy of the data was developed to meet ASPRS Class 1, large-scale, map accuracy standards.  All data is in the Georgia State Plane Coordinate System, West Zone, NAD83. All digital orthophotography is delivered in a MrSID file format, with the imagery undergoing a 30:1 compression.  Contour data and planimetric mapping (transportation and hydrography) are delivered in either ESRI’s .E00 file format, which can be imported into ESRI’s coverage format, or DXF format.

Recommended usage: The Cobb County Base Map data is developed for 100’ scale mapping. The data should be used for preliminary planning purposes only. A field survey should be conducted before any construction or other activities take place.

Aerial Photography:
The photography is taken from an aircraft flying approximately 4700’ above the ground, with thousands of individual overlapping color photographs being produced.  The photographs are then converted to digital images for the direct viewing, in a computer, of features as small as manholes and fire hydrants.

Specifications:

• Flown at an altitude of 4700’
• Photo Endlap 60%, Sidelap 40%
  
• Camera: Leica ADS40 pushbroom scan digital aerial camera/sensor system
• Camera tilt and crab: not more than 3 degrees

Digital Orthophotos:
Most digital orthophoto imagery is captured from aerial photography.  New images are rectified to remove distortion so the images are at a consistent map scale. Three types of distortion are removed. Distortion caused by the camera angle and distortion caused by the camera lens. A digital terrain model is used to remove distortion caused by a landscape’s terrain. A typical orthophoto project consists of hundreds of photographs that have been rectified and mosaicked together. Mosaicking involves balancing adjacent photographs’ radiometry, which refers to an image’s color, brightness and contrast.

Specifications:

• Aerial photo scale: 1” = 600’
• Map scale: 1” = 100’
• Accuracy: +- 2’
• Pixel resolution: .5’
• Tiles: 5000’ x 5000’
• Number of Tiles: 478
• File size: 10MB, 30:1 compression
• File format: Georeferenced MrSID
• Data delivery: DVD or CD

Other features:

• Seamless mosaicking (appearance/edge matching)
• Radiometric/tone balancing
• Building lean reduction
• Bridge and railroad adjustment

Limited Planimetrics:
Limited planimetrics are selected surface features digitally mapped from aerial photography. Cobb County requested that edge of pavement, street centerlines, and hydrography be mapped to meet ASPRS Class 1, large-scale, map accuracy standards.

Specifications:
Transportation
Road Centerlines
Driveway Centerlines
(200 ft minimum length)
Edge of Pavement _

1. All graded roads, gravel-surfaced or otherwise improved, shall be collected.
2. Edge of pavements and road centerlines will not be classified. Public vs. private shall not be determined. Paved vs. unpaved shall not be distinguished.
3. Unimproved roads (logging roads, jeep trails) shall not be collected. Parking lots and golf course cart paths are not to be mapped.
4. Bridges shall be mapped – centerlines and edge of pavements shall run straight across bridges, over/under passes, and streams.
5. Hidden road lines shall be collected.
6. Driveways over 200 feet long shall be mapped. Only the centerline of driveways will be shown. A driveway is an improved vehicular traveled way that leads from a road to one or more structures.
7. Do not use fillets at intersections. The edge of pavement at these intersections shall be collected.
8. Road centerlines and driveway centerlines will be uniquely identified.
9. Centerlines and edge of pavements will not be attributed with names.
10. The actual edge of pavement (EOP) will be mapped. Copy parallel from one side to the other is unacceptable. Compile the road as a continuous feature. When a road crosses a model boundary the linestrings shall be mathematically snapped in 3D. For the centerlines of the roads at both scales each segment of centerline from one intersection to the next shall be a separate element. The joins from one centerline element to the next shall be a 3D mathematical snap.

Hydrography
Standing Bodies of Water
Rivers
Streams
Closing Line

1. All waterways shall be mapped as streams and show a continuous network.
2. All clearly defined channels shall be mapped even though they have no water showing. Err on the side of too much mapping of streams, not too little.
3. Standing bodies of water shall be mapped as closed polygons.
4. Rivers/double line streams shall be mapped as closed polygons.
5. Waterways over 25 feet wide shall be mapped as double-lines (closed polygons) while those less than 25 feet wide shall be mapped as single-line features at both scales.
6. Artificial closing lines shall be collected and included wherever a double-line stream/river and a lake/pond join.
7. All standing bodies of water shall be collected as closed polygons with no type distinction required.
8. The watercourses in the stereo model shall be captured as best determined from the lowest point in the model to the highest point.
9. For any hydro feature which is obscured by a bridge or some other man-made feature, a hidden feature shall be captured in a manner that ensures connectivity of the down-stream to up-stream drainage and completes the required polygon for standing bodies of water.

Contours:
Contours are a pattern of lines, with each line drawn through points of equal elevation. These line patterns follow the contour of the landform. Cobb County has requested contour lines that meet ASPRS Class 1, large-scale, map accuracy standards for 2’ contour intervals.

Obscure Areas:
Obscure Areas designate areas where, due to heavy tree canopy or other obstructions, the mapped features were unable to be collected directly from the orthophotography. All mapped features (transportation, hydro, and 2’ contours) contained within the Obscure Areas should be considered an approximation of the true feature.

Click here to Buy Data Tiles Online



<< Back to Top