SPATIAL DATA INPUT

Introduction

A major proportion of the effort in any GIS project is assembling the data in digital form, and creating a spatial database in which all maps, images and spatial data tables are properly geocoded and in spatial register.

The topics covered in this chapter are related to spatial data capture and conversion. These topics are:

Data sources

Map projections

Digitizing

Coordinate conversion

Data sources

Data sources can be classified as primary or

secondary and digital or non-digital

Primary data are data that have been collected

from the site as observations in its raw form

Primary, non-digital data such as field mapping,

hand-recorded data

Primary, digital data such as geochemical data and

remote sensing images

Data sources

Secondary data: when data are interpreted, edited

and processed for use by others, they become

secondary data sources

Secondary, non-digital data such as maps and

tables

Secondary, digital data such as digital database

Map projections

Map projections represent the round earth on a flat

medium such as a sheet of paper or a computer

screen

Define the spatial relationship between locations on

earth and their relative locations on a flat map

Are mathematical expressions which transform the

spherical earth to a flat map

Cause the distortion of one or more map properties

(scale, distance, direction, shape)

Map projections

The location of a spatial entity on the earth’s surface is defined in mathematical terms using either geographical (global) coordinates, or planar coordinates according to some projection.

It is possible for a GIS to store and manipulate all spatial data in geographical coordinates (latitudes and longitudes).

All spatial data are visualized in planar coordinates (paper, monitors), thus most GIS use planar map projections for storing spatial coordinates, in order to avoid the repeated transformation from geographic to projection coordinates.

Map projection

geographic coordinates

Geographic coordinates are expressed in terms of latitude and longitude

A line joining the north (N) and south (S) pole of the globe through some point is called a meridian

The latitude of the same point measures the angle, φ , between the point and the equator along the meridian.

The longitude measures the angle, λ , between the meridian through the point and the central meridian (Greenwich, England) in the plane of the equator.

Planes passing through the earth but not intersecting the center form small circles at the earth’s surface

Meridians are thus great circles

Lines of constant latitude, called parallels, are small circles, except the equator which is a great circle

Map projection

geographic coordinates

Map projection

plane coordinates

Locations on a plane are defined by Polar or

Cartesian coordinates

X

Y

P

O

Map projection

Geometric distortion

Projection transformations from the globe to a

plane introduce geometric distortion

Projections can be classified according to their

geometric distortion characteristics into conformal

(equiangular), equal area, and equidistant types

Conformal projections

Preserve angular relationships between features

Parallels and meridians cross at right angles

Small areas remain relatively undistorted

Map projection

Geometric distortion

Equal area projection

Preserve areas but at the expense of angular relationship

It is useful for representing point distributions over large

regions, because point density is unaffected

Equidistant projections

Preserve neither angular nor area relationship but distance

relationships in certain directions are maintained

It is often used in atlases covering large regions because

they are a compromise between the other two projections

Map projections

figure of the earth

To define projections mathematically, a geometrical model known as the figure of the earth is used to generate projections.

The simplest models are the plane and the sphere.

The realistic model is the spheroid, which is a figure produced by rotating an ellipse about the minor axis

The radius of the earth is 1 part in 300 shorter at the pole than the equator

The spheroid can be precisely defined by the lengths of the semi-major(equatorial) and the semi-minor(polar) axes, a and b, respectively.

The flatting ,f, and the eccentricity ,e, of the spheroid terms used in some of the transformation equations, are defined by:

f = (a – b)/a e2 = 2f – f2

Map projections

figure of the earth

Map projections

figure of the earth

Map projections

developable surfaces

Projections can be classified into planar (azimuthal), conic, and cylindrical types depending on the shape of the developable surface

These surfaces can be visualized as flat, cone-shaped or cylindrical, touching or cutting the globe in one of six ways.

In the tangent case, the developable surface touches the globe along a great circle for a cylinder, or along a small circle for a cone, or at a point for a plane.

For the secant case, the developable surface cuts the globe.

Forward transformation is the transformation from the geographic coordinates to the planar coordinates .

The inverse transformation is the transformation from the planar coordinates to the geographic coordinates

Map projections

The Universal Transverse Mercator (UTM) system

The UTM grid utilizes the transvers Mercator projections, which results from wrapping the cylinder round the poles instead of round the mercator, as for the ordinary Mercator projection.

The central meridian is the meridian where the globe touches the sphere

The globe is divided in to 60 UTM zones, numbered from west to east, starting from zone 1 at 180W.

Each zone is 6 degrees of longitude wide

It extends from84N to 80S.

the origin of each zone is the intersection of the central meridian at the equator

Displacements in the x and y directions are called UTM eastings and UTM northings.

A UTM spatial reference requires three numbers, the easting, the northing and the zone number

UTM

Planar surface

Cylindrical surface

Conic surface

Coordinate conversion

In GIS projects we have to bring all the data from the coordinate systems in which they are currently occur to a new uniform planar coordinate system

This requires a sequence of coordinate conversions

In GIS projects

We choose the geographical extents of the region to be studied

Select a suitable map projection as the working projection

All the maps have to be converted from their projection to the geographic coordinates, then converted to the working projection

Coordinate conversion

Vector conversion

Case source (A) with table coordinates from

digitizing

Table coordinates are converted to projection

coordinates using control points and coefficient for

affine transform

The projection coordinates are converted to geographic

coordinates (inverse transformation) using the map

projection parameters

Geographic coordinates are converted the coordinates

of the working projection using projection parameters

Coordinate conversion

Vector conversion

Case (B) digital vector input (X,Y)

The projection coordinates are converted to geographic coordinates (inverse transformation) using the map projection parameters

Geographic coordinates are converted to the coordinates of the working projection using projection parameters

Case (C) digital vector input (φ, λ) Geographic coordinates are converted to the

coordinates of the working projection using projection parameters