CEE 8813 · GIS Lecture 1 · Data models

Data Model Lab

A GIS stores the world in one of two ways. The vector model keeps each county, road, curve and crash as exact X, Y vertices, linked to one row of attributes. The raster model lays a grid over the map and keeps one value per cell. Here are the GDOT District 7 layers from the lab in both models. Click a feature to see its vertices and its record, then switch to Raster and change the cell size to see what the grid keeps and what it loses.

Map · GDOT District 7 · drawn in projected feet

Vector

Drag to pan, scroll or pinch to zoom. Click a feature to select it. In Raster, click a cell to read its values.

Feature anatomy

Polygon

Attribute table

What you are seeing

Shapefile anatomy: the lab's real files

A shapefile is a set of files with one base name. Pick one of the four shapefiles from the lab folder, then untick a file to see what ArcGIS Pro does when it is missing. Sizes are the bytes on disk; bars use a log scale.

Opens normally

Try this in class

  1. Click a state route on the map, or press “Select a route” below. Count its vertices (squares) and find the two nodes at its ends. Then compare the length the page measured from the vertices with TMEAS − FMEAS in its attribute row.
  2. Switch to Raster at 1,000 ft. Zoom into the Fulton–DeKalb line with “Outline the vector features” on, and watch the county edge turn into a staircase. Drag the cell size to 250 ft, then to 1 mi.
  3. In Raster the page picks the cell with the most crashes. Read how many crashes merged into it, switch back to Vector, and click the crashes there one by one. Each one is its own record with its own severity.
  4. Watch the Vector and Raster sizes while you move the cell size. At which cell size does the raster become larger than the vector? Switch the pixel type to 32-bit and check again.
  5. Open the Crashes table and hover the column names. Which CSV header became F__Serious, and which became Manner_of_?
  6. In Shapefile anatomy, untick the .prj of Collision, then the .shx. Which one still opens? Divide the .shx size minus 100 by 8 and compare with the record count.

Pitfalls students hit

  • Emailing or copying only the .shp. ArcGIS Pro needs at least the .shp, .shx and .dbf, and without the .prj the coordinate system is unknown.
  • Renaming one file of the set in File Explorer. The files must share a base name, so rename the shapefile in the Catalog pane.
  • Looking for a field called “# of Fatalities”. After the export to shapefile it is F__of_Fata, and queries must use that name.
  • Treating the AREA field as the polygon's area. In GA_Counties it is FEMA's land area, so it is smaller than the area of the geometry, which also covers lakes and rivers.
  • Thinking a raster “has” the crashes. A crash-count raster keeps how many crashes fall in each cell and nothing else about them.

Back to Lecture 1: GIS data

Slides 16 to 26 introduce the two data models, the parts of a vector feature, and the files ArcGIS keeps them in. Each card points to the part of this page that shows it with the lab data.

Vector vs rasterSlides 16–17

A data model is how real-world objects become digital. Vector keeps exact shapes; raster keeps a grid of values. Slide 17 asks which picture is the vector one; the Raster button answers it with the lab's own layers.

ArcGIS Pro: Polygon to Raster, Polyline to Raster and Point to Raster do this conversion with a cell size you choose. The crash grid here is what Point to Raster gives with the Count option.

Points, lines, polygonsSlide 18

The three vector objects are called features, and each is stored as a series of X, Y coordinates. A line's vertices give its shape; the nodes are its two end points.

On the map: squares are vertices, dots are the two nodes of the selected line.

Feature classes and attributesSlides 19–21

Like features are grouped into feature classes, and each feature's attributes sit in one row of a table. Coordinates are stored in a coordinate system; this page measures in feet, like State Plane.

ArcGIS Pro: right-click a layer, Attribute Table. Selecting a row selects the feature, and the other way round.

Shapefiles and geodatabasesSlides 22–26

A shapefile stores geometry and attributes for one geometry type in several files. A geodatabase holds feature classes, feature datasets and tables in one database: personal, file or enterprise (SDE).

ArcMap's ArcCatalog is now the Catalog pane in ArcGIS Pro.

Neither model is better. Vector suits features with exact edges and many attributes, such as roads, curves and crash reports. Raster suits values that vary everywhere, such as elevation, land cover or a crash density surface. The cell size decides how much of the world the grid can still see.