1. Lecture 2 – Spatial Data Preparation
Before we can create a GIS project, we need to collect and process data, which including spatial data, and attribute data.
Several important issues during this process:
Scale
Map projection
Geo-reference
Data conversion
2018/11/8
Jun Liang, UNC

2. Jun Liang, Geography @ UNC
2-1 Scale
Spatial resolution is critical to GIS application.
Several factors you may consider when you decide your spatial data resolution:
Your research area
Other data sets
Software and hardware
Example:
Hydrology modeling input layers – land-use map, remote sensing images, and climatic factors.
2018/11/8
Jun Liang, UNC

3. Jun Liang, Geography @ UNC
2-2 Map projection
Different ways of projecting could be produced by surrounding the globe in a cylindrical fashion, as a cone, or even as a flat piece of paper. Projections onto these three surfaces are called cylindrical, conic, and azimuthal, respectively.
Note: No map projection can preserve both conformality and equal area.
2018/11/8
Jun Liang, UNC

4. 2-2 Map projection (Cont.)
Two major projection steps:
Select a reference globe – converts the actual globe to a reference globe.
Select a projection – the reference globe is mathematically projected onto the flat surface.
Datum (ellipsoid) – Clark1866/NAD27(North American Datum), NAD83/GRS80 (Geodetic reference system), WGS84 (world geodetic system, a modified version of GRS80)
2018/11/8
Jun Liang, UNC

5. 2-2 Map projection (Cont.)
The most common coordinates systems for USA:
UTM – Universal Transverse Mercator
UTM, which has been included since the late 1950s on most USGS topographic maps. UTM is often used for accurate mapping.
(2) SPC – State Plane Coordinate System
State plan coordinate system (SPC) was devised by the US Coast and Geodetic Survey in 1930S. SPC uses either a transverse Mercator or Lambert’s conformal conic projection.
2018/11/8
Jun Liang, UNC

6. 2-2 Map projection (Cont.)
Map projection conversion
Parameters you map need for converting map projection A to map projection B:
Coordinate systems (UTM, SPC, World, Gauss Kruger, etc.)
Datum (Nad27, Nad83, Nad83-feet, etc.)
Projection Zone (OHIO north, OHIO south, North Carolina, etc.)
Standard latitude/longitude lines
Projection
2018/11/8
Jun Liang, UNC

7. Jun Liang, Geography @ UNC
2-3 Geo-reference
Georeferencing is the process of scaling, rotating, translating and transforming the spatial data to match a particular size and position, or a controlled coordinate system.
Why we need geo-referencing spatial data?
Old maps (projection and origins unknown and can not be retrieved.
Aerial photos
Scanned maps
2018/11/8
Jun Liang, UNC

8. 2-3 Geo-reference (Cont.)
Geo-referencing Raster Map
Raster image – made up of pixels; no particular size. Without projection coordinate system information (cell size, projection, coordinates, etc.), it will not be displayed correctly in GIS software.
To register a raster image to a specific coordinate system, you will need to choose a projection for your data frame that will most closely resemble the geometry of your map. GCPs (Ground Control Point) should also carefully selected to register aerial photos.
2018/11/8
Jun Liang, UNC

9. 2-3 Geo-reference (Cont.)
Vector data can also be geo-referenced. (when?)
Steps of geo-referencing in ArcGIS:
There are two ways you can geo-reference your raster image: (1) by assigning coordinates that you type in as taken from a paper map or (2) by aligning it with a target layer that has a coordinate system. General steps:
Add the raster image (and any target layers you want to align it to.)
Add control points that link known raster positions to know target data positions in map coordinates
Save the geo-referencing information with the raster image as an .aux file
2018/11/8
Jun Liang, UNC

10. 2-3 Geo-reference (Cont.)
Transform/Warp
When you created enough GCP links, you can transform your raster dataset to permanently match the map coordinates of the target data. It uses a polynomial transformation to determine the correct map coordinate location for each cell in the raster.
More GCPs can increase the overall accuracy of the transformation.
GCPs’ quality are also important.
2018/11/8
Jun Liang, UNC

11. Jun Liang, Geography @ UNC
2-4 Data Conversion
Raster to Raster
Raster to Vector
Vector to Vector
Examples: jpg->tiff, img->jpg/tiff, img->grd
grd->shapefile, img->shapefile
coverage->shapefile
dxf->shapefile
2018/11/8
Jun Liang, UNC