Digital Spatial Data

Francisco Olivera, Ph.D., P.E.Department of Civil Engineering

Texas A&M University

Definitions

Digital Spatial Datasets: Synthesis -- in electronic format -- of geographic (map) and tabular (table) information.

Data structure: method used to organize and store digital spatial datasets.

Data Types

FeaturesPoint datasetsLine datasetsPolygon datasets

SurfacesGrid datasetsTIN datasetsImages datasets

NetworksSimple dentritic networksSimple loop networks

Features

All points required to define the feature geometry have to be stored as part of the feature definition.

Features

Point datasets

One-to-one relation between features in the map and records in the

table.

Features

Line datasets

One-to-one relation between features in the map and records in the

table.

Features

Polygon datasets

One-to-one relation between features in the map and records

in the table.

Surfaces

Grid datasets

SurfacesTIN datasets

Surfaces

Image datasets

Networks

Simple dentritic networks

Networks

Simple loop networks

Data Structures of Features

Topologic data structures:Store (1) the geometry of the features, and (2) the spatial relationship between connecting or adjacent features (i.e., topology) in tabular format. Points do not coincide.Lines are simple.Polygons are simple and space-filling (i.e., no overlaps or empty spaces).Shared polygon boundaries are stored only once.Coverages have topologic data structures.

Cartographic data structures:Stores the geometry of the features.Points can coincide.Lines can be complex.Polygons can be complex, and not necessarily space-filling.Shared polygon boundaries are stored as part of the definition of each of the adjacent polygons.Shapefiles have cartographic data structures.

Data Structure of Features

A line is an open sequence of points in which the first and last points are called nodes, and the remaining intermediate points are called vertices.

Nodes

Vertices

Data Structure of FeaturesSimple linesComplex lines

Data Structure of Features

Simple polygonsComplex polygons

Data Structure of Features

Space-filling polygonsNot space-filling polygons

Data Structure of Features

CB

A

D

1

2

3 4

5

I

II

III

Line From- Node To- Node Right Polygon Left Polygon

1 B A I I I I

2 D A I I I I

3 B D I I I

4 D C I I I I I

5 C B I I I I I

Line and polygon topology

Data Structure of Surfaces

Grid datasets:Cellular-based data structure composed of square cells of equal size arranged in rows and columns.

Grid definition requires: (1) the coordinates of the upper-left corner, (2) the cell size, (3) the number of rows, (4) the number of columns, and (5) the value at each cell.

Cells that do not store any value are called NODATA cells.

Number of columns

Num

ber

of

row

s

Cell size

(x, y)

Triangular Irregular Network (TIN) Datasets:Dataset constructed by connecting points -- for which the TIN parameter is known – forming triangles.Triangle sides are constructed by connecting adjacent points so that the minimum angle of each triangle is maximized.Triangle sides cannot cross breaklines.The TIN format is efficient to store data because the resolution adjusts to the parameter spatial variability.

Data Structure of Surfaces

Triangular Irregular Network (TIN) Datasets

Data Structure of Surfaces

Data Structure of Surfaces

Image datasets:ARC Digitized Raster Graphics (ADRG)Windows bitmap images (BMP) [.bmp]Multiband (BSQ, BIL and BIP) and single band images [.bsq, .bil and bip]ERDAS [.lan and .gis]ESRI Grid datasetsIMAGINE [.img]IMPELL Bitmaps [.rlc]Image catalogsJPEG [.jpg]MrSID [.sid]National Image Transfer Format (NITF)Sun rasterfiles [.rs, .ras and .sun]Tag Image File Format (TIFF) [.tiff, .tif and .tff]TIFF/LZW

Data Structure of Networks

Simple dentritic networks:System of simple lines -- called links -- connected at their nodes.

All links point towards (or away-from) a given node called sink.

One-to-one relation between From-Node (or To-Node) and link.

A

D

BC

E

F

12

3

4

5

F is the network sink.

Link From- Node To- Node

1 A D

2 B D

3 C E

4 D E

5 E F

Data Structure of Networks

Simple loop networks:System of simple lines -- called links -- connected at their nodes. Links can point in either direction or in both directions.

A B

E

I

J

K

D

C

G

H

1

F

1

2 3

45

6

78

9

1011

12

Storing Datasets

FeaturesCoverages are stored partially in their own folder and partially in the common INFO folder.Shapefiles are stored in at least three files (with extensions .shp, .shx, .dbf) and up to seven files (with extensions .sbx, .sbn, .ain and .aih).

SurfacesGrid and TIN datasets are stored partially in their own folder and partially in the common INFO folder.Image datasets are stored in different ways depending on the image format.

NetworksSimple dentritic and loop networks are stores as a line dataset of links, a point dataset of nodes, and a topology file.

Storing Datasets

Structure of a folder containing different types digital spatial data.

Coverage

Grid

TIN

Image.tif

Shapefile.shpShapefile.shxShapefile.dbf

Info

Managing Datasets

RenamingAlways use ArcInfo or ArcView utilities to rename coverages, shapefiles, grids and TINs because some information is internally stored with the dataset name.Images can be renamed using the operating system utilities.

Copying and MovingAlways use ArcInfo or ArcView utilities to copy and move coverages, grids and TINs to make sure the information stored in the INFO folder is included.Shapefiles and images can be moved or copied using the operating system utilities, making sure all the files are included.

Sharing Datasets

Interchange files

Coverages, grids and TINs are shared as interchange files.

An interchange file is a single file -- with extension E00 -- that includes all information stored in the dataset folder and its share of information contained in the INFO folder.

If a limit is set on the size of the interchange file, then several smaller files (i.e., E00, E01, E02, …) are generated rather than one single file. This option was common when storage media had limited capacity.

An interchange file is obtained by exporting a coverage, grid or TIN. In turn, a coverage, grid or TIN is obtained by importing an interchange file.

Compressed (“zipped”) files

To make sure that all files are included, shapefiles and images can be shared as compressed files.