Projection, Georeferencing and Spatial Adjustment CAD

Projection

A map projection is a method for taking the curved surface of the Earth (3D) and displaying it on a flat surface (2D). A projected coordinate system is always based on a geographic coordinate system. The below table shows a few key differences between a GCS and PCS.

GCS (Geographic Coordinate System) PCS (Projected Coordinate System)
3D Spheroid Surface 2D Flat Surface
Latitude and Longitude XY Locations
Datum Map Projection

Map projections are designed for specific purposes. Conformal projections preserve the shape of the features, Equal area projections preserve the area of the feature displayed, Equidistant projections preserve the distance between certain features on a map while the Azimuthal projection maintains the directions of all points on the map.

Common Errors Due to Incorrectly set Coordinate Systems

 

Quick Easy Steps for Projecting

  1. Enquire with the source owner of the data, research or decide which coordinate system the data should be assigned. If the original coordinate system of the data cannot be sourced, it will be your responsibility to assign the correct coordinate system to the data.
  2. Before deciding which coordinate system the data should be assigned; visually assess the data or the layer properties of the data, to check whether the data is originally in GCS (Geographic Coordinate System) or PCS (Projected Coordinate System). Add the data in the Map Window in ArcMap.
    extent
    Take note of the Extent that has large numbers 361332,327979dd. This indicates that this is a Projected Coordinate System.

    decimal degrees extent
    Notice the Extent has small numbers -26,333041?? starting with to decimals. This indicates that they are Decimal Degree measurement which is a Geographic Coordinate System
  3. Define the coordinate of your data by using the Define Projection tool (This tool is for datasets that have an unknown or undefined coordinate system defined)define projection
  4. After defining the coordinate system of the data, check against a Basemap if it is located at the correct place.add basemap
  5. Project the data using the Projection Tool (This tool is to change the dataset from one coordinate system to another)project tool

 

Georeferencing and Spatial Adjustment

 

Georeferencing

 

Provides a correct, real-world spatial reference to Raster or CAD datasets; which is either missing a real-world spatial reference or has an unknown spatial reference.

Georeferencing is the process of aligning geographic data to a reference dataset in a known coordinate system. This method helps to associate a physical map, raster or CAD document with a spatial location. When you Georeferencing a dataset, you define where the data is located using map coordinates. Georeferencing uses Control Points, that associates the data with a specific location on earth; which allow the georeferenced dataset to be viewed, queried and analysed with other geographic data.

During Georeferencing it is important to use correct Reference Datasets; raster or vector feature classes can be used as reference data only if the data has the correct spatial reference. Identify distinctive locations that are visible in both datasets, these will be used as Control Points. The control points link the original dataset that is being georeferenced to the reference data –  the first control point is plotted on the original dataset (data that needs to be aligned with the reference data) the second control point is then plotted on the corresponding location on the reference data. Corresponding Links are established from the control points, which will be used to align the original dataset with the reference data.

Spatial Adjustment

Like Georeferencing; Spatial Adjustment aligns the original dataset to a reference data, based on links between corresponding control points. The major difference between the two methods is the original datasets and the usage of the method; Georeferencing is used to re-create a missing or unknown spatial reference for Raster or CAD data while Spatial Adjustment is used to correct the alignment of editable vector data.

Data in GIS usually comes from different sources, which means the user is required to perform additional work to integrate and use the data together. Spatial adjustment is used to correct; inconsistencies between data sources, correct geometric distortions and align features together. There are a variety of adjustment methods that can be used to adjust all editable data sources. Another interesting task in spatial adjustment is the ability to transfer attributes from one feature to another.

There are three methods for performing spatial adjustment: transformation, edgematching and rubber sheeting.  The edgematching method is typically used for connecting the end points of features with each other, rubber sheeting is best used for aligning minor geometric adjustments; this method stretches, shrinks and reorients features to match the reference data and the transformation method is like the transformation method used in georeferencing; it will shift, scale, rotate and skew the data if necessary.

 

The table below shows the major differences between the two methods above.

Georeferencing Spatial Adjustment
The process of aligning data with missing or unknown spatial reference to reference data in a known coordinate system Editing functionality for aligning data with a spatially accurate reference dataset
Transformation Method Transformation Method

Edgematching Method

Rubber Sheeting Method

 

Works Out of the Edit session Works in the Edit Session
CAD, Raster Imagery, Aerial Imagery Feature Class or Shapefile (editable vector data)

Common Errors Due to Inconsistency in Data

georef_cad
Error showing an un-georeferenced CAD Layer
inconsitency
Errors showing Features Classes (vector data) that do not align to the Reference Data

 

Quick Easy Steps for Correctly Aligning CAD/DWG Data

  1. Assign GCS (Geographic Coordinate System) to the CAD/DWG file (WGS_1984) in ArcCatalog, if it has no spatial coordinate system / unknown coordinate system.unknown coordinates
  2. Load a Basemap for reference purposes, it is advisable to choose the South Africa Cadastre Basemap available on ArcGIS Online Basemap especially if you are using CAD/DWG Files.basemap
  3. Georeference the CAD/DWG file, using the reference data (data in the correct geographic location that has the correct Spatial Reference i.e. farm portions).georef
  4. When Georeferencing; it is vital to have two distinctive locations in both the CAD/DWG File and the Reference Data, these two distinctive locations will be used as Control Points.georef_control_points
  5. Export the georeferenced CAD Feature to a Feature Class place it in a file geodatabase once georeferenced, first create a new file geodatabase if necessary. Working with Feature Classes is recommended especially if you must spatially adjust the data after Georeferencing.
  6. Adjust the feature class by using Spatial Adjustment (use the Affine method)spatialadjustment
  7. Use the Define Projection tool to define a coordinate for the feature class that perfectly aligns with the Reference data.
  8. Project, it accordingly and assess the resultsresults

Limitations of Georeferencing CAD datasets

Georeferencing a CAD dataset is limited to one- and two-point transformations using the similarity transformation method:

  • one-point transformation comprises one link and moves the dataset
  • two-point transformation comprises two links and moves, rotates, and scales the dataset uniformly

Both methods preserve the shape and angles of the CAD dataset, however, the aspect ratio (the ratio of the width to the height of an image or screen) of the CAD drawing is distorted

Spatial Adjustment of CAD datasets

The spatial adjustment method maintains the aspect ratio of the CAD drawing and prevents skewing to the x- and y- axes. However, it should be noted there will be an inherent deformation of the aspect ratio, from the georeferencing step.

 

 

 

 

Contributor: Busisiwe Ngobe

 

Understanding Survey Diagrams

Surveying is all about measuring distances, angles and positions on or near the surface of the earth. Surveyors use mathematical techniques to analyse field data. Survey measurement relies on understanding two basic scientific, accuracy and reliability.

Types

  • Plane surveying: Earth surface is considered a 2D plane with x-y dimensions.
  • Geodetic surveying: Earth surface is considered spherical (ellipsoid) 3 dimensional

Classes

  1. Preliminary survey (data gathering): is the gathering of data (distances, position and angles) to locate physical features (rivers, roads and other structures) so that data can be plotted to scale on a map or plan, also include the difference in elevation so that contour could be plotted.
  2. Layout survey: Marking on the ground (using sticks, iron bar or concrete monuments) the features shown on a design plan features: – Property lines (subdivision survey). – Engineering work (construction survey). – Z-dimensions are given for x-y directions.
  3. Control survey: used to reference preliminary and layout surveys.
  • Horizontal control: arbitrary line tied to the property line or HWY centre or coordinated control stations.
  • Vertical control: Benchmarks: points whose elevation above sea level is defined accurately.

Different methods of surveying

  1. Topographic survey: preliminary surveys used to tie earth surface features.
  2. Hydrographic survey: preliminary surveys tie underwater feature to surface control line
  3. Route surveys: preliminary, layout and control surveys that range over a narrow but long strip of land (highways, railroads, electricity transmission lines and channels).
  4. Aerial survey: preliminary and final surveys to convert an aerial photograph into scale map using photogrammetric techniques.
  5. Construction survey: layout of engineering work.
  6. Built survey: preliminary surveys tie in features that just have been constructed
  7. Property surveys: preliminary, layout and control surveys determine boundary locations.

Unit of measurement

There are two main measuring systems:

English system and Metric system (SI units).

  • Angles are measured by: Degrees, minutes and seconds.
  • 1 revolution = 360 degrees, 1 degree = 60 minutes and 1 minutes = 60 seconds

http://faculty.kfupm.edu.sa/CE/hawahab/WEBPAGE/CE260/NOTES/1CE260%20CH%20%201.pdf

Types of survey diagram

  • Servitude diagrams (powerlines, pipelines or municipal services) for registering servitudes over an existing property.
  • Lease diagrams for registering long leases over portions of properties.
  • Consolidation diagrams when it is required to consolidate several individual properties into one, taking out certificates of consolidated title.
  • Mineral diagrams to register mineral rights separately from the land rights.
  • Mining title diagrams for registering the right to extract minerals from the land.

http://csg.dla.gov.za/diagram.htm

Coordinate systems

SA_Coordinate

Angles of direction

Directions are measured clockwise which is the opposite for GIS data (counter clockwise).

Azimuths

  • Azimuths are horizontal angles measured clockwise from any reference meridian.
  • In a plane surveying, azimuths are generally measured from north/south starting at 0⁰.
  • Azimuths are used advantageously in the boundary, topographic, control and other kinds of survey, as well as in computations.

 

In the south azimuth system, the angles are measured clockwise from south 0⁰.

south azimuth system

 

ArcGIS by default, accept angular measurements of the polar direction measuring system

Polar angles are measured counter-clockwise from the positive x-axis, east 0⁰.

c_clockwise

Bearing

  • Assumes that total station is set up at points A, B, C, D, E, F and G; bearing read on lines AB, BC, CD, DE, EF and FG.
  • AB, BC, CD, DE, EF and FG are forward bearings and the inverse will give backwards bearings
  • Both the forward and backwards bearing should have the same numerical values.

One can change the direction measuring system and angular units the editing tools use on the Units tab of the Editing Options dialog box.

http://moodle.najah.edu/pluginfile.php/47169/mod_resource/content/0/Angles_Azimuths_Bearings.pdf

http://desktop.arcgis.com/en/arcmap/10.3/manage-data/editing-fundamentals/about-direction-measuring-systems-and-units.htm

 

 

 

 

Contributor: Lutho Mbeki

How to maintain data connections in your MXD

Software: ArcGIS Desktop
Platform: Windows

Problem: Broken links

Answer:

Often you may need to repair data sources and broken links in your MXDs. The idea of fixing this problem manually can be tedious, particularly when having data from multiple sources. ArcMap has an ability to store the pathname to the data within the map document (MXD) so when you reopen your map document, ArcMap locates the data using these stored pathnames. By default, ArcMap references the data source using absolute file or full paths. A full path begins with a drive letter followed by a colon, such as D: An example of an absolute full path is: Y:\GIS\data\DB\Provinces.shp.

The problem with absolute paths is when you share or move the MXDS, everyone who uses the map must have the data on their computer exactly the same folder structure. If not you are likely to get the following error

Broken links

ArcMap provides an option to set Data sources in case the links are broken, you can manually reset the paths to your data.

Set Sources

The best practice, however, is to store relative paths to a current directory in your ArcMap. Relative paths make use of two special symbols, a dot (.) and a double-dot (..), which translate into the current directory and the parent directory. Where your pathnames would be saved as: ..\GIS\data\DB\Provinces.shp.

This option points ArcMap to the location of the data contained in the map relative to the current location on disk of the map document itself.  This also allows you to easily move the map and its data to a different hard drive on your computer, or give the map and its data to another person to copy onto their computer without having to change the file paths.

Here is how to save your MXD with relative paths:

  1. Open ArcMap
  2. Open you MXD
  3. Click on the File menu.
  4. Select the Map Properties option.
  5. File Menu
  6. Click on the Data Source Option button.
  7. Change the radio button to “Store relative path names”.
  8. Store Relative Paths
  9. Click OK.
  10. Click OK Customise Menu and select ArcMap Options.
  11. Customize Menu
  12. Check the Make relative paths the default for new map documents radio button.
  13. Arcmap Options

Note: To benefit from relative paths, you have to move the MXD and its data. Therefore, if you copy its parent folder with the MXD, its data and any the sub-folders, your data sources will be maintained. Now you don’t need to do the exercise of going to the file menu and changing the Map Document Properties. All new MXDs will be set to relative paths. Note that existing ones will still need to be changed in the Map Document Properties.