Buffer

Create a buffer around a map point and display the results as a graphic.

Use case

Creating buffers is a core concept in GIS proximity analysis that allows you to visualize and locate geographic features contained within a polygon. For example, suppose you wanted to visualize areas of your city where alcohol sales are prohibited because they are within 500 meters of a school. The first step in this proximity analysis would be to generate 500 meter buffer polygons around all schools in the city. Any such businesses you find inside one of the resulting polygons are violating the law.

How to use the sample

Tap the map when the sample loads. A planar buffer (brown) and geodesic buffer (green) will be created at the tap location using the distance (miles) in the text box specified by the slider. Continue tapping to create additional buffers. Notice that buffers closer to the equator appear similar in size. As you move north or south from the equator, however, the geodesic polygons become much larger. Geodesic polygons are in fact a better representation of the true shape and size of the buffer. Tap "Clear All" to remove all buffers and start again.

How it works

1. The map AGSPoint for a tap on the display is captured.
2. The static method class AGSGeometryEngine.bufferGeometry(_:byDistance:) is called to create a planar buffer polygon from the map location and distance.
3. Another static method, class AGSGeometryEngine.geodeticBufferGeometry(_:distance:distanceUnit:maxDeviation:curveType:) is called to create a geodesic buffer polygon using the same inputs.
4. The polygon results (and tap location) are displayed in the map view with different symbols in order to highlight the difference between the buffer techniques due to the spatial reference used in the planar calculation.

Relevant API

• AGSGraphicsOverlay
• class AGSGeometryEngine.bufferGeometry(_:byDistance:)
• class AGSGeometryEngine.geodeticBufferGeometry(_:distance:distanceUnit:maxDeviation:curveType:)

Additional information

Buffers can be generated as either planar (flat - coordinate space of the map's spatial reference) or geodesic (technique that considers the curved shape of the Earth's surface, which is generally a more accurate representation). In general, distortion in the map increases as you move away from the standard parallels of the spatial reference's projection. This map is in Web Mercator so areas near the equator are the most accurate. As you move the buffer location north or south from that line, you'll see a greater difference in the polygon size and shape. Planar operations are generally faster, but performance improvement may only be noticeable for large operations (buffering a great number or complex geometry).

For more information about using buffer analysis, see the topic How Buffer (Analysis) works in the ArcGIS Pro documentation.

Tags

analysis, buffer, euclidean, geodesic, geometry, planar

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// Copyright 2018 Esri.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import UIKit

protocol BufferOptionsViewControllerDelegate: AnyObject {
    func bufferOptionsViewController(_ bufferOptionsViewController: BufferOptionsViewController, bufferDistanceChangedTo bufferDistance: Measurement<UnitLength>)
}

class BufferOptionsViewController: UITableViewController {
    @IBOutlet private weak var distanceSlider: UISlider?
    @IBOutlet private weak var distanceLabel: UILabel?

    weak var delegate: BufferOptionsViewControllerDelegate?

    private let measurementFormatter: MeasurementFormatter = {
        // use a measurement formatter so the value is automatically localized
        let formatter = MeasurementFormatter()
        // don't show decimal places
        formatter.numberFormatter.maximumFractionDigits = 0
        return formatter
    }()

    var bufferDistance = Measurement(value: 0, unit: UnitLength.miles) {
        didSet {
            updateUIForBufferRadius()
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        updateUIForBufferRadius()
    }

    private func updateUIForBufferRadius() {
        // update the slider and label to match the buffer distance
        distanceSlider?.value = Float(bufferDistance.value)
        distanceLabel?.text = measurementFormatter.string(from: bufferDistance)
    }

    @IBAction func bufferSliderAction(_ sender: UISlider) {
        // update the buffer distance for the slider value
        bufferDistance.value = Double(sender.value)
        // notify the delegate with the new value
        delegate?.bufferOptionsViewController(self, bufferDistanceChangedTo: bufferDistance)
    }
}