Working With Projections.

Practical Guide: Working with Projections in ArcMap

1. Introduction

Understanding and working with map projections is essential in GIS, as they affect how spatial data is represented on a flat surface. This guide will walk you through the steps of working with projections in ArcMap, including defining, projecting, and analyzing spatial data.

2. Understanding Projections

• Projection: A method for representing the curved surface of the Earth on a flat map.
• Coordinate Systems: Geographic (latitude/longitude) and projected (flat, Cartesian coordinates).

3. Checking Data Projections

A. Load Your Data

1. Open ArcMap: Launch ArcMap and create a new project.
2. Add Data: Click on the Add Data button and load your shapefiles or raster data.

B. Check Layer Properties

1. Right-Click on the Layer: Select Properties.
2. View Source Tab: Click on the Source tab to see the coordinate system of the layer.

4. Defining the Projection

If your data lacks a defined projection, you need to define it.

A. Define Projection

1. Open ArcToolbox: Click on the ArcToolbox icon.
2. Navigate to Data Management Tools:

• Expand Projections and Transformations.
• Select Define Projection.

1. Select Input Dataset: Choose the layer that needs a defined projection.
2. Select Coordinate System:

• Click on the Select button.
• Choose the appropriate coordinate system from the list (e.g., UTM, State Plane).

1. Click OK: Apply the changes.

5. Projecting Data

To ensure all data layers are in the same projection, you may need to project them.

A. Project Tool

1. Open ArcToolbox: Click on the ArcToolbox icon.
2. Navigate to Data Management Tools:

• Expand Projections and Transformations.
• Select Project.

1. Select Input Dataset: Choose the layer you want to project.
2. Choose Output Coordinate System:

• Click on the Output Coordinate System box.
• Select the desired projection (e.g., NAD83, WGS84).

1. Specify Output Location: Choose where to save the projected layer.
2. Click OK: Run the tool to create the projected dataset.

6. Working with Different Projections

When working with multiple layers, ensure they are all in the same projection.

A. Reprojecting Layers on the Fly

1. Set Data Frame Projection:

• Right-click on the Layers in the Table of Contents.
• Select Properties.
• Go to the Coordinate System tab.
• Choose the desired coordinate system for the data frame.
• Click OK.

1. Automatic Reprojection: ArcMap will automatically reproject the layers to match the data frame’s coordinate system.

7. Analyzing Data with Different Projections

Conduct spatial analysis while ensuring all layers are in the same projection.

A. Perform Analysis

1. Use Tools: Access tools like Buffer, Clip, or Intersect from the Geoprocessing menu.
2. Select Layers: Ensure that all input layers are in the same projection for accurate results.

8. Saving Your Project

1. Save Your Work:

• Click File > Save As to save your project (.mxd file) for future editing.

9. Conclusion

You have successfully learned how to work with projections in ArcMap, including defining, projecting, and analyzing spatial data. Understanding projections is vital for accurate spatial analysis and map-making.

• Define a projection : To define a projection for a CAD dataset, you can:
  1. Open the Arc Toolbox pane
  2. Click Data Management Tools > Projections and Transformations > Define Projection
  3. Select the CAD dataset in the Input Dataset or Feature Class section
  4. Click the browse icon in the Coordinate System section
  5. Select the coordinate system for the CAD dataset in the Spatial Reference window
  6. Click OK in the Define Projection window
  7. Rename the projection file with the same name as the CAD dataset, but with the extension .prj
• Change the projection of a dataframe: To change the projection of a dataframe, you can:
  1. Right-click in the map and select Properties > Coordinate System
  2. Select the Coordinate System tab
  3. Choose a coordinate system from the tree or import one from a data source
  4. Click OK
• Create a custom projected coordinate system :To create a custom projected coordinate system, you can:
  1. Double-click Border in the Contents pane to open the Map Properties window
  2. Click the Add Coordinate System button on the Coordinate Systems tab
  3. Choose New suggested projected coordinate system
  4. Type a name for the coordinate system
  5. Set the geographic coordinate system
  6. Click Map Neatline under Get extent from
  7. Choose a projection property