Vector Analysis

1. Data Management Functions vs. Data Analysis Functions

2. Three Vector Data Management Functions

If asked to explain strict data management operations, use these three:

1. Data Conversion: Transforming spatial data between different models or formats, such as converting a vector shapefile into a raster grid (Rasterization) or converting CAD files into GIS shapefiles.
2. Data Editing: Modifying existing vector features to correct errors or update outdated spatial and attribute data. This includes moving vertices, reshaping polygons, or updating the text fields in the attribute table.
3. Topology Building/Validation: Establishing and checking the spatial relationships between adjacent or connected features to ensure data integrity. This involves applying rules like "polygons must not have gaps" or "lines must connect at nodes" to clean up digitizing errors.

3. Detailed Geoprocessing and Vector Functions

The examiners frequently ask you to explain these specific functions along with their spatial maps and attribute table effects.

A. Clip Asked 9 times

Years: 2080, 2079, 2075 Magh, 2075 Bhadra, 2074 Magh, 2074 Bhadra, 2073, 2072, 2071

• Concept: Acts like a "cookie cutter." It extracts features from an input layer that fall strictly within the spatial boundary of a clip layer.
• Spatial Map: If the Input is a large irregular polygon (e.g., a forest) and the Clip layer is a perfect circle (e.g., a study area), the Output is only the piece of the forest that fits inside the circle.
• Attribute Table: The output layer retains only the attributes of the Input layer. The Clip layer's attributes are ignored completely.

Exam Example

Attribute Table Example — Clip

B. Intersect Asked 9 times

Years: 2082, 2080, 2079, 2075 Magh, 2075 Bhadra, 2074 Magh, 2074 Bhadra, 2072, 2071

• Concept: Computes the geometric intersection of two layers. Only those features that overlap in both layers are preserved in the output.
• Spatial Map: If Polygon A (Land Use) overlaps Polygon B (Flood Zone), the Output is strictly the overlapping middle section.
• Attribute Table: The output attribute table joins the attribute columns from both Polygon A and Polygon B for the overlapping region.

Exam Example

Attribute Table Example — Intersect

C. Union Asked 8 times

Years: 2082, 2080, 2079, 2075 Magh, 2074 Magh, 2074 Bhadra, 2072, 2071

• What it does: It takes two overlapping polygon layers and physically cuts them against each other. Every place where boundaries cross, a completely new, smaller polygon is created.
• The Attribute Table: It joins the columns of Table A and Table B together (horizontally). Every new polygon knows what it was in Layer A and what it was in Layer B.
• Rule: Both input layers must be polygons.
• Civil Engineering Example: You have a polygon layer of "Soil Types" and another polygon layer of "Land Use" for the exact same watershed. You use Union to find every unique combination of soil and land use. If an "Agricultural" polygon overlaps a "Clay Soil" polygon, the Union operation cuts out that specific overlapping shape and its attribute table will read: Land Use: Agricultural | Soil: Clay.

Exam Example

Attribute Table Example — Union

D. Dissolve Asked 6 times

Years: 2082, 2079, 2075 Magh, 2075 Bhadra, 2074 Bhadra, 2071

• Concept: Aggregates adjacent features or polygons that share the same attribute value into a single, larger feature.
• Spatial Map: If you have 10 adjacent small square parcels, and 8 of them share the attribute "Residential", the output merges those 8 squares into one large "Residential" polygon boundary.
• Attribute Table: The records are aggregated. Multiple rows are combined into a single record, reducing the total number of rows in the table.

Exam Example

Attribute Table Example — Dissolve

E. Buffer Asked 4 times

Years: 2082, 2080, 2079, 2075 Magh

• Concept: Creates a proximity zone (a new polygon) around a map feature (point, line, or polygon) measured at a specific uniform or variable distance.
• Spatial Map: A line representing a highway will output a thick polygon tracing that highway at a 50m distance on both sides.
• Attribute Table: A new polygon feature is created. The attribute table typically inherits the base feature's attributes and adds a new column stating the buffer distance (e.g., "BUFF_DIST = 50").

Exam Example

Attribute Table Example — Buffer

F. Merge Asked 4 times

Years: 2075 Bhadra, 2074 Magh, 2074 Bhadra, 2072

• What it does: It takes two completely separate datasets (usually adjacent to each other) and groups them into one single file. It does not calculate intersections or break polygons into smaller pieces.
• The Attribute Table: It simply takes the rows from Table B and adds them to the bottom of Table A (stacking them vertically).
• Rule: The input layers must be the same geometry type (e.g., you can only Merge lines with lines, or polygons with polygons).
• Civil Engineering Example: You have a shapefile of the "Kathmandu Road Network" and a separate shapefile of the "Lalitpur Road Network." You use Merge to create a single shapefile called "Valley Road Network."

Exam Example

Attribute Table Example — Merge

G. Erase Asked 1 time

Years: 2073

• Concept: Removes portions of the input feature that overlap with the "erase" feature. It is the geometric opposite of a Clip.
• Spatial Map: If the Input is a large polygon and the Erase layer is a smaller overlapping circle, the Output is the large polygon with a blank "hole" punched out where the circle was.
• Attribute Table: Retains only the input layer's attributes; the records and geometry for the erased portions are permanently removed.

Exam Example

Attribute Table Example — Erase

H. Map Join / Spatial Join Asked 1 time

Years: 2073

• What it does: It transfers attributes from one layer to another based purely on their spatial relationship (location) (like "intersects," "is completely within," or "is closest to"). It does not alter the geometric shapes of the target layer at all.
• The Attribute Table: It joins the columns of Table B to the side of Table A (widening it horizontally) based on physical location.
• Rule: The input layers can be different geometry types (e.g., you can join Polygon attributes to Point features).
• Engineering Example: For your final year project on Site Response Analysis, you have a point shapefile containing 50 individual soil borehole locations from Kathmandu Geo Lab. You also have a polygon shapefile of the Kathmandu municipal wards. If you use a Spatial Join, the software looks at where each borehole point falls physically, and automatically adds the "Ward Number" and "Ward Population" columns from the polygon map directly into your borehole point attribute table.

Exam Example

Attribute Table Example — Spatial Join

I. Update Asked 1 time

Years: 2073

• Concept: Computes the geometric intersection and replaces the input layer's features and attributes with the update layer's features and attributes where they overlap.
• Spatial Map: A base map of land use, and a smaller polygon of a "New Airport Development". The Output shows the base map, but the area under the new development is entirely replaced by the new polygon's shape.
• Attribute Table: The attributes of the base layer are overwritten by the attributes of the update layer purely in the overlapping region.

Exam Example

Attribute Table Example — Update

J. Split Asked 1 time

Years: 2073

• Concept: Breaks a single input layer into multiple separate output files using the boundaries of a split layer.
• Spatial Map: A map of rivers crossing through three different district polygons. The Output results in three completely separate files (e.g., River_DistA.shp, River_DistB.shp, River_DistC.shp).
• Attribute Table: The original attribute table is divided into multiple smaller tables, one for each new output file.

Exam Example

Attribute Table Example — Split

4. Differentiating Specific Functions

Clip vs. Intersect

Merge vs. Dissolve

5. Prove that Intersection = Union + Clip

This mathematical/logical proof requires you to break down the operations step-by-step:

1. Let Layer A be the Input Layer and Layer B be the secondary layer.
2. The Union Step: Performing a Union on Layer A and Layer B results in a combined map containing three distinct geometric areas: A unique, B unique, and the Overlap (A ∩ B). Crucially, the attribute table now contains the joined columns of both A and B.
3. The Clip Step: Take the output of the Union and Clip it using Layer B's exact boundary as the cookie-cutter.
4. Clipping to Layer B removes anything outside of B (deleting A unique). We are left with only the area inside B. Since the problem asks for the interaction of A and B, clipping the unioned layer strictly to the overlapping zone isolates the spatial geometry of (A ∩ B).
5. Conclusion: The final geometry is just the overlapping area. Because the first step was a Union, the attribute table contains the joined data from both A and B. A geometric overlap that contains the joined attributes of both input layers is the exact definition of an Intersection. Therefore, Union + Clip = Intersection.

1. The Theory Question

Question: Define the term spatial analysis. Explain different types of vector overlay operations in GIS. Also explain three examples of neighborhood functions. [2+4+4]

A. Definition of Spatial Analysis (2 Marks)

Spatial analysis is the core analytical process in GIS where we extract or create new information about a set of geographic features. It involves evaluating suitability, predicting outcomes, measuring proximities, and modeling relationships between spatial data to solve complex engineering and planning problems.

B. Types of Vector Overlay Operations (4 Marks)

Vector overlay involves combining two or more separate polygon layers to create a completely new layer containing both the spatial boundaries and the attribute data of the inputs.

Intersect (The "AND" Operator): Keeps only the overlapping regions of the input layers. The output attribute table combines data from both.

Union (The "OR" Operator): Keeps the entire extent of all input layers. Polygons are split wherever lines cross, and the attribute table joins all data.

Identity / Erase: Identity keeps the base map's boundaries but adds the overlay's attributes. Erase completely removes the overlapping portions from the base map.

C. Three Examples of Neighborhood Functions (4 Marks)

Neighborhood functions analyze the relationship between a target feature and its surrounding area.

1. Buffering: Creating a zone of a specified distance around a point, line, or polygon (e.g., establishing a 100m no-build zone around a river).
2. Near / Distance Analysis: Calculating the exact shortest distance from one feature to another (e.g., finding the distance from a proposed school to the nearest existing hospital).
3. Thiessen Polygons (Voronoi Diagrams): Creating boundaries around a set of points so that any location inside a given polygon is closer to that polygon's central point than to any other point (e.g., mapping the service areas for different rainfall stations).

Landfill Site Selection Workflow

Question: Write about the necessary tools and explain how you will perform the following analyses using GIS: Best location for landfill area so that following constraints are fulfilled: Houses > 1000m, River > 1500m, Road < 500m, Water bodies > 1500m. Provided data: Shapefile of river, land use data, point data of the houses, shapefile of roads, polygon shapefile of the study area. [10]

Here is the precise step-by-step workflow you need to write:

Step 1: Buffer the Restricted Features (Tool: Buffer)

Since a landfill is hazardous, it must be far from people and water. We buffer the restrictions:

• Buffer the "Houses" point layer by 1,000m.
• Buffer the "River" line layer by 1,500m.
• Buffer the "Water bodies" polygon layer by 1,500m.

Step 2: Combine the Restricted Zones (Tool: Union or Merge)

Use the Union tool to combine the three buffers from Step 1 into a single, massive "Restricted Area" polygon layer.

Step 3: Extract the Available Land (Tool: Erase)

Take the total "Study Area" polygon as the input feature, and use the "Restricted Area" from Step 2 as the erase feature. The output is a map of the "Available Land" where it is legally safe to build.

Step 4: Buffer the Favorable Proximity Feature (Tool: Buffer)

The site must be accessible for garbage trucks. Buffer the "Road" line shapefile by 500m to create the "Accessibility Zone".

Step 5: Isolate the Final Suitable Sites (Tool: Intersect)

Intersect the "Available Land" (from Step 3) with the 500m "Road Buffer" (from Step 4). The resulting polygons are the exact locations that meet every single distance constraint.

Step 6: Filter by Appropriate Land Use (Tool: Select by Attributes)

Using the provided "Land use data" shapefile, perform a Spatial Join with your final sites. Then, run an SQL query (e.g., SELECT * WHERE Land_Use = 'Barren') to ensure the landfill isn't placed on top of agricultural or forested land.

(Make sure to sketch a simple conceptual flowchart or a mock map showing a road with a buffer overlapping an available land polygon to secure the full 10 marks!)

Flood Plain Analysis Workflow

This is a multi-step vector analysis problem that uses Buffer, Intersect, and Select operations to answer three related flood-risk questions.

Part (a): Locate Settlements Within 1 km of Rivers

1. Buffer the river line shapefile by 1,000 m (1 km) on both sides. Output: "River_Buffer_1km" polygon.
2. Intersect (or Clip) the settlement point data with the River_Buffer_1km polygon.
3. Output: Only the settlement points that fall inside the 1 km river buffer — these are the at-risk settlements.

Part (b): Flood Plain Zoning

Flood plain zoning classifies land adjacent to rivers into risk zones based on distance.

1. Create multiple buffers around the river at different distances:
   • 0–500 m → High Risk Zone (Zone A — Floodway)
   • 500–1,000 m → Moderate Risk Zone (Zone B — Flood Fringe)
   • 1,000–2,000 m → Low Risk Zone (Zone C)
2. Erase the smaller buffer from each larger one to create distinct ring-shaped zones (e.g., 1000m buffer ERASE 500m buffer = Zone B only).
3. Alternatively, use Multiple Ring Buffer tool (available in ArcGIS/QGIS) to create all zones at once.
4. Optionally intersect with DEM-derived low-elevation areas to refine the flood plain boundary using actual topography rather than distance alone.

Part (c): Agricultural Land in Flood Plain

1. Take the flood plain polygon from Part (b) (e.g., the combined High + Moderate risk zones, or the 1 km buffer).
2. Intersect the flood plain polygon with the land use polygon shapefile. The output contains all land use parcels that fall within the flood plain, with attributes from both layers.
3. Select by Attributes: Run SQL query SELECT * WHERE Land_Use = 'Agriculture' on the intersected output.
4. Output: Polygons showing only the agricultural land that lies within the flood plain — this is the cropland at risk of flood damage.
5. Optionally, calculate the total area: use the Calculate Geometry tool to sum the area of all selected agricultural polygons.

Summary of Tools Used

Figure-Based Question: Clip, Buffer, Intersect, Union

Figure below shows 2 roads, 7 buildings as points, and the area of interest (AOI) as polygon layers. (a) Sketch the road and building layers after clipping with the AOI polygon (assume clip-road and clip-bld are the outputs respectively). (b) Draw the 50 m buffer for each of the clipped layers along with clip-road and clip-bld layers (assume buffered layers are named as buf-road and buf-bld respectively). (c) Sketch the intersection of buf-road and buf-bld and also show the output fields. (d) Sketch the union of buff-road and buf-bld and also show the output fields.

(a) Sketch the clipped layers (2 Marks)

• What to draw: 1. Draw a dashed rectangle representing the Area of Interest (AOI). 2. Inside this rectangle, draw the roads (lines) and buildings (dots) strictly stopping at the dashed boundary. 3. Crucial detail: Do not draw any lines or dots outside the AOI. The Clip tool acts as a cookie-cutter, completely discarding anything outside the AOI polygon.
• Labels: Label the lines as clip-road and the dots as clip-bld.

(b) Draw the 50m Buffers (2 Marks)

• What to draw: 1. Redraw your clip-road lines and clip-bld dots from part (a). 2. Around every building dot, draw a circle (this is your 50m point buffer). 3. Along every road line, draw a thick, parallel boundary on both sides (this is your 50m line buffer). 4. Crucial detail: Make sure some of the building circles overlap with the road buffers, as this is required for the next steps!
• Labels: Label the circular polygons as buf-bld and the road polygons as buf-road.

(c) Sketch the Intersection and show output fields (3 Marks)

• What to draw: 1. Look at where your buf-road polygon crosses over your buf-bld circles. 2. Draw only the specific overlapping, lens-shaped areas where both buffers exist at the exact same time. Erase everything else.
• The Output Fields (Attribute Table): Because an Intersect joins the tables of both input layers, the output table must contain the columns of both the roads and the buildings.
  • Draw a table with these columns: [FID] | [Shape] | [Road_fid] | [Road_name] | [Road_type] | [Bldg_fid] | [Bldg_name]
  • Note: The question provided these specific fields for the road and buildings, so you must write them out to get the marks.

(d) Sketch the Union and show output fields (3 Marks)

• What to draw: 1. Draw the entirety of both buffer layers from part (b). Draw all the road buffers and all the building circles. 2. However, everywhere a circle crosses a road buffer, draw a distinct solid line to show that a new, smaller polygon has been created by the overlapping boundaries.
• The Output Fields (Attribute Table): A Union also joins the tables of both layers.
  • Draw a table with the exact same columns as part (c): [FID] | [Shape] | [Road_fid] | [Road_name] | [Road_type] | [Bldg_fid] | [Bldg_name]
  • Crucial detail for extra marks: Write a quick note below the table stating: "Where a building buffer does not overlap with a road buffer, the road attribute fields will contain 'Null' values, and vice versa."

End of Chapter 6 – Vector Analysis