GEOG 380 - Lab 06

Projected Coordinate Systems

What is a geographic coordinate system (GCS)?

A system that locates places on Earth using latitude and longitude, usually in decimal degrees.

What is a projected coordinate system (PCS)?

A flat map-based coordinate system created from a projection, usually using linear units like metres.

What is the main difference between a GCS and a PCS?

A GCS uses angular coordinates on a curved Earth, while a PCS uses flat coordinates on a projected map.

What is a map projection?

A mathematical method for transforming the curved Earth onto a flat surface.

Why are map projections necessary?

Because the Earth is curved, but maps are flat.

What is the main downside of all map projections?

Every projection introduces distortion

What types of distortion can projections cause?

Distortion of shape, area, distance, and direction.

What does a conformal projection preserve?

Local shape and angles.

What does an equal-area projection preserve?

Relative area

What does an equidistant projection preserve?

Certain distances, but not all distances everywhere.

What is a compromise projection?

A projection that tries to balance distortions without perfectly preserving one property.

What are the main projection families?

Cylindrical, conic, and azimuthal (planar).

What kind of projection is Lambert Conformal Conic?

A conformal conic projection

What type of regions is a Lambert Conformal Conic projection often good for?

Midlatitude regions that extend more east-west than north-south.

What is an on-the-fly projection?

A display change where ArcGIS redraws the data in a new projection without changing the original dataset.

Does on-the-fly projection change the underlying data?

No, it only changes how the data is displayed.

What does the Project tool do?

It creates a new dataset with coordinates rewritten into a new projected coordinate system.

Why might you project a dataset instead of relying on on-the-fly projection?

For faster drawing and more reliable spatial measurements.

What is a central meridian?

The reference longitude at the centre of many map projections.

What can changing the central meridian do?

It can recenter the projection and shift where distortion is minimized.

What are standard parallels?

Lines of latitude where a conic projection touches or cuts the Earth.

Why are standard parallels important?

Distortion is usually lowest near them and increases farther away.

What is scale factor in a projection?

A value that affects how map scale is applied across the projection.

Why does scale factor matter?

Because it can make planar distances larger or smaller than true distances.

Why can the same city have different projected coordinates in different maps?

Because each projection creates a different flat coordinate grid.

Why should a city’s decimal-degree location stay nearly the same across maps?

Because it represents the same real-world location in the geographic coordinate system.

Why might decimal-degree readings differ slightly between maps?

Cursor placement, snapping, rounding, or zoom level differences.

What is a definition query?

A filter that displays only features meeting a chosen condition.

Does a definition query delete data?

No, it only controls which features are shown.

What is an attribute join?

A way to attach fields from one table to another using a shared attribute.

Why use an attribute join in mapping?

To bring needed information into another layer for queries, symbology, or analysis.

Does an attribute join merge geometry?

No, it only adds attribute information.

Why might you export or copy features after a join?

To save the joined attributes into a new feature class and avoid rebuilding the join.

Why are regional projections often better for regional maps?

Because they minimize distortion in the area of interest.

Why can a regional projection badly distort the rest of the world?

Because it is optimized for one region, not the whole globe.

What is geodesic distance?

The shortest distance measured on the curved Earth or ellipsoid.

What is planar distance?

Distance measured on the flat projected map.

Why is geodesic distance usually the same across different map projections?

Because it is based on the Earth model, not the flat map grid.

Why can planar distance change from one projection to another?

Because each projection distorts the flat map differently.

Which is generally better for true Earth-surface distance: geodesic or planar?

Geodesic

Why can two maps have the same planar distance measurement?

Because they use the same projected coordinate framework.

Why is projection choice important for thematic maps?

Because projection affects how data patterns are visually interpreted.

What should you consider when choosing a projection for a thematic map?

The map purpose, geographic extent, and whether shape, area, or distance matters most.

When is a geographic transformation needed?

When changing the geographic coordinate system or datum.

Was a geographic transformation needed in this lab when staying in WGS84?

No

What does the Preserve Shape option do during projection?

It adds vertices to help maintain feature geometry more accurately.

Why might projected data draw faster than on-the-fly projected data?

Because the coordinates are already stored in the target system, so less redrawing work is needed.

What is the key trade-off in all projection selection?

You reduce some distortions at the expense of others.

Why is there no perfect world map projection?

Because a curved Earth cannot be flattened without distortion.

What is one major lesson from this lab?

Projection choice and projection parameters strongly affect map appearance, coordinates, and measurements.