Projections, Scale and
Metadata
Important Elements of
Digital Maps
1. Map
Projections and Coordinate Systems
All the data layers you use must match up
correctly for you to draw them on top of each other or combine them to see
relationships. This means they must be in the same map projection and
coordinate system.
Several issues are involved in choosing a map
projection and coordinate system, including where on the globe the area you are
mapping is located, how large the area is, and whether you need precise
measurements of distance or areal extent.
1.1. What is a
Coordinate System?
A coordinate system specifies the units used
to locate features in two-dimensional space and the origin point of those
units. Latitude and longitude is a coordinate system (often called the
"geographic" coordinate system).
If you are using an established GIS database,
chances are the data is already in the same coordinate system and projection.
If you are collecting data from various sources, though, you will want to
verify this.
A sphere or ellipsoid represents Earth
surface:
1.2. What are
parallels and meridians?
Parallels are circles (or ellipses) that
result from intersections of earth surface with planes perpendicular to earth
axis. Meridians are circles that result from intersections of earth surface
with planes with that contain the earth axis. The longest parallel is called equator. The meridian that passes from
Greenwich Observatory in England is considered to be the prime meridian.
1.3. Latitude
and Longitude:
· Latitude of a point is the distance or angle (arc on a sphere or ellipsoid that represents earth) from the equator to the parallel that contains the point
· Longitude of a point is the distance or angle (arc on a sphere or ellipsoid that represents earth) from the prime meridian to the meridian that contains the point
1.4. What is a
map projection?
A map projection translates the locations on
the globe onto the flat surface of your map. All map projections distort the
shapes of the features being displayed to some degree, as well as measurements
of area, distance, and direction. (This is why on some maps of the earth,
Greenland appears to be almost as big as North America.). If you are mapping a
relatively small area such as a town or county, this distortion is usually
negligible. The larger the area mapped, the more this distortion will affect
you.
Illustration: Mercator is a cylindrical
projection and Lambert is a conic projection:
Mercator Projection
Lambert Conic Projection
In our maps we are using UTM (Universal
Transverse Mercator) projection.
Exercise 1 - Where is Spain? Where is
Cordoba?
·
Open
an empty Arcmap document
·
Add
data 
·
Select
C:\arcgis\arcexe82\Metadata\Data\WorldCountries.shp
·
Locate
Spain on the map by zooming in and out
·
Add
data 
·
Select
C:\arcgis\arcexe82\Metadata\Data\EuropeUrbanAreas.shp
·
Right
click the last layer (EuropeUrbanAreas) - Label Features
·
Locate
Cordoba on the map by zooming in and out
·
Add
data 
·
Select
one of the Century layers that you created from the zip disk
·
Notice
that your features are not found within the extent of Cordoba
The reason that your features are not within
the area of Cordoba is because we have not specified a projection or coordinate
system for our layers. Arcmap doesn't know where to place your Century layer
with regard to the world map.
Exercise 2 - Specify a projection and
coordinate system for your layers
·
Remove
the Century layer from the Arcmap document
·
Open
ArcCatalog 
·
Right
click on one of you Century layers/Properties
·
Select
Fields tab
·
In
the table under Data Type select Geometry
·
Under
Field Properties next to Spatial Reference select 
·
Select
Coordinate System tab and then Import[1]
·
Browse
to T:\International\Cordoba\Cordoba_GIS\ 34and26.shp and click Add
·
We
have imported a projection for our layer (look at Name and Details)
·
Click
OK
·
In
Arcmap, add 
the century layer to the frame. Notice your
features are aligned correctly with the world map and within the Cordoba urban
area
·
Repeat
the procedure in ArcCatalog for all you Century layers. They should all have a
projection and coordinate reference system.
Exercise 3 - Enter a point with known
coordinates in the map
·
Firstly,
we create a point shapefile in ArcCatalog
·
Right
click on the zip drive and select New – Shapefile
·
Name
the shapefile MY_POINT
·
Choose
Feature Type POINT and click OK
·
Create
the fields in the Attribute table:
- Select your MY_POINT shapefile and click on the Preview tab
- At bottom select Preview-Table
- Select Options-Add Field
- Enter the data:
- Name: Name
- Type: Text
- Size: 55
·
Import
a coordinate system for MY_POINT from the base map as explained in Exercise 2
·
Close
ArcCatalog
·
In
Arcmap open your document from the zip drive (Base Map Cordoba)
·
Add
the World map to your document
(C:\arcgis\arcexe82\Metadata\Data\WorldCountries.shp)
·
Next
we change the projection for the whole map:
- Layers (Right Click)/Properties/Coordinate System/Import
- Browse to The WorldCountries.shp
·
Add
your MY_POINT shapefile to your document
·
Edit
the MY_POINT layer and add a point:
- Select the MY_POINT layer
- Editor/Start Editing
-
Select the Create
New Feature tool 
- Right Click on the screen/ Absolute X, Y
- Type -4.7784, 37.8810 and enter
- The point should be displayed within the Mezquita Catedral.
- Editor/Stop Editing and save the edits
2. Scale
2.1. What is
scale?
The map scale is a ratio between a distance
on the map with the corresponding distance on the earth's surface. The distance
in the map is usually expressed as a unit of one, therefore map scales are
stated as a fraction (for example: 1:25,000).
The notion of scale represented by a fraction
is somewhat misleading because of the distortion involved when projecting the
three-dimensional earth's surface to a two-dimensional plane. As a result of
these distortions the scale is not uniform throughout the map. We can think of
the average scale in a map as the ratio between the radius of a mini globe and
the earth's radius. In fact, the fractional scale represents this average scale
that is also known as the principal scale of a map. In general we can classify
maps as large scale (1:100 to 1:2,000), intermediate scale (1:2,000 to
1:10,000) and small scale (1:10,000 to 1:100,000,000). These classifications
differ considerably depending on the organization and the use of maps.
2.2. Why scale
is important?
·
Map Scale determines the appearance of features: Most features can be represented as more than one shape. The scale of a map tells how the size of the
map features compares with the size of the geographic objects they represent. The
larger the map scale, the bigger the map features will appear. Depending on the
map scale, a feature such as a city can appear as a point or as an area, and a
feature such as a river can appear as a line or as an area. For example, the
Mohawk River is represented as an area on the Schenectady map, but on a map of
New York State it appears as a line. The city of Schenectady covers the entire
area shown on the downtown Schenectady map, but the same area appears as a
single point on the state map. The buildings that are points on the Schenectady
map would appear as areas on a larger-scale land-use map.
·
Map scale determines how much detail we need in a map: The scale of the map determines how big or how small features appear
and how much detail you can show. If your audience needs to see a lot of detail
in a coastline, the scale must be large enough to show it. A sailor needs to
see every cove and harbor to stay on course. A wildlife biologist needs to see
all the granulations in the coastline--its inlets and peninsulas--to identify
and preserve good habitat areas.
2.3. How can
we change the scale in a digital map?
·
One
way to control the amount of detail is by zooming
in and zooming out.
·
Zooming in enlarges the scale so you can see more detail.
·
Zooming out decreases the scale and the size of the features. Features like
harbors, inlets, and offshore islands become too small to see.
Exercise 4 - Explore the map scale
display in Arcmap:
·
Zoom
in and out looking at the map scale
·
Manually
change the map scale and see the increase or decrease of detail
·
Type
the scale 1:2000 (the original scale of the base map). Notice the appropriate
level of detail in the base map
3. Metadata
3.1 What is
Metadata?
Metadata is frequently described as
"data about data." Metadata is additional information (besides the
spatial and tabular data) that is required to make the data useful. It is
information you need to know in order to use the data. The information
contained in metadata will document the creation of a dataset and give you an
idea of what the cartographic product to which it is attached was designed to
do. Metadata represents a set of characteristics about the data that are
normally not contained within the data itself. Metadata could include:
·
An
inventory of existing data
·
Definitions
of the names and data items
·
A
keyword list of names and definitions
·
An
index of the inventory and the keyword list for access
·
A
record of the steps performed on the data including how it was collected
·
Documentation
of the data structures and data models used
·
A
recording of the steps used on the data for analysis
3.2. Why Is
Metadata Important?
Spatial metadata is important because it not
only describes what the data is, but it can reduce the size of spatial data
sets.
Spatial data also supports software-based and
organizationwide standards. The benefit of having software-based data standards
is that the program is easier to use, and users can readily move data between
systems and platforms. By creating metadata, you are creating a standard in
naming, defining, cataloging, and operating standards for all departments. This
in turn is a vital foundation for understanding, collaborating, and sharing
resources with others.
Spatial metadata is important because it
supports easier spatial data access and management. Metadata provides a guide
to the casual and novice user's question, "How do I know what to ask
for?" Metadata can provide information on what is available in an area of
interest, where the information is, how current it is, what format it is in,
and what use constraints apply. For spatial data professionals, metadata
provides feature- and attribute item-level metadata management. This way,
updates are easily accommodated and integrated into daily use of the data.
Metadata is not an end in itself; it is a tool that will greatly improve your
work with spatial data and increase your overall GIS benefits.
Exercise 5 - Create metadata for your Century layers:
·
Open
ArcCatalog 
·
On
the left side find your zip disk and click on one of your Century layers
·
On
the right side click on Metadata 
·
Click
on Edit Metadata icon 
·
Fill
in:
-
Identification/General/Abstract: Summary of the project
-
Identification/General/Purpose: Purpose of the project
-
Identification/Contact/Details: Your names and contact
information
-
Data Quality/Source
Information/General/Source Scale Denominator: 2000
·
Explore
Spatial Reference/ General and Spatial Reference/Horizontal Coordinate
system. You will notice the projection and coordinate system we defined in
the previous exercise
·
When
you are done with metadata dialog click Save
·
Now
we can import the metadata we created to other Century layers:
-
On
the left side locate and click the next Century layer
-
On
the right side click on Metadata 
-
Click
on Import Metadata icon 
-
In
Format choose XML
-
Browse to your zip disk and find
the metadata you created <your
layer>.shp.xml
-
Click
Open and then OK
-
Repeat
this procedure for the rest of your century layers.