Geographic Information Systems (GIS) Services: FAQ & Troubleshooting

What is a Geoid

A hypothetical model of the Earth using mean sea level as a base. It represents the form the earth's oceans would take if there were no land and the water was free to respond to the earth's gravitational and centrifugal forces.

What is an Ellipsoid or Spheroid?

A mathematical model for representing the earth. Specifically, it is a three-dimensional shape created from a two-dimensional ellipse.

Why are ellipsoids used rather than geoids in developing map projections?

1. Ellipsoids are based on a single set of spherical trigonometry equations which are comparatively mathematically simple.
2. Geoids are defined by geographically local transformations rather than a single transformation and thus, more computationally demanding.

What is a Datum?

A datum (horizontal/vertical) is an ellipsoid that provides the base elevation for mapping or defining a location. It is usually defined by:

• Origin – represented as X, Y, & Z angles from the center of earth mass
• Orientation – the rotation angle
• Size/Shape – the ellipsoid in terms of semi-major and semi-minor axes

What is a Coordinate System?

A reference system for representing the exact locations of geographic features on Earth. A Coordinate system allows people to determine area, distance, direction, and shape. Every coordinate system is defined by the following:

• Its measurement framework (either geographic or plan metric)
• A unit of measurement (Angular e.g. Degrees Minutes Seconds (DMS) / Decimal Degree (DD) or Linear e.g. Feet or Meters)
• Other measurement system properties e.g. Datum etc.

What is a Map Projection?

A process of representing the curved 3D surface of the earth on a flat 2D surface.

How is a location estimated or defined?

A location is estimated as: Latitude and longitude on the 3D surface earth or by X, Y Cartesian coordinates on a 2D map.

How do these concepts: Map Projection, Coordinate System, and Datum work together?

The Earth is a 3D surface with a myriad of features located on it. In defining the location of the features, geodetic engineers and surveyors first had to define the shape of the earth. The most fitting model for the earth’s shape is the ellipsoid. Based on the ellipsoid (in conjunction with the two poles), a reference system comprising spherical coordinates (longitude and latitude) is superimposed on the earth surface. The reference system forms part of a broader system called the Coordinate System. The intersection of any of the two coordinates defines the location of any feature present there in the real world. This definition of a location occurs within a 3-dimensional context such as a globe. A globe, however, has small scales thus can provide very little detail of the earth. They are also expensive and inconvenient to carry, and could only provide a limited view of the earth at a glance. To overcome these challenges, scholars came up with a process/method to transform the 3D surface unto a 2D surface. The process of doing that is called Map projection.

The above explanation describes the case of global mapping. In cases involving local mapping, some modification takes place. Since an ellipsoid approximates the shape of the earth, most often spatial properties of some local areas are either exaggerated or underestimated. As a result, a local ellipsoid (which best approximates the shape of the area) is derived to accommodate such discrepancies. Such local ellipsoids are however built on the global ellipsoid. The local ellipsoids are often referred to as a datum.

Distinguish between spherical coordinates and Cartesian coordinates?

Both are used to define locations of geographic features. However, spherical coordinates have some serious drawbacks:

1. They are inappropriate for spatial measurement; due to the variable length of one degree of longitude across the Earth, spherical coordinates do not provide a reliable measure for determining length/distance or area.
2. The units are angular (eg. DD, DMS) and therefore can be confusing to understand by a lay person.
3. Inconsistent comparison.

To deal with the above issues, the Cartesian coordinate system is applied. This converts all angular units into X and Y linear units e.g. feets or meters thus makes it easy to comprehend. The Cartesian coordinate system is only applicable after a map projection has been implemented.

What is a spatial reference?

A spatial reference is a series of parameters that define the coordinate system and other spatial properties for a dataset particularly within a geodatabase. It is typical that all datasets from the same location (placed in the same geodatabase) use a common spatial reference definition. A spatial reference includes the following:

• A Coordinate System
• The Coordinate precision (often referred to as the Coordinate Resolution)
• Processing tolerances (such as the cluster tolerance)
• The spatial extent covered by the dataset (often referred to as the Spatial Domain)

What is a Geographic Coordinate System (GCS)?

A reference system that uses latitude and longitude to define the locations of points on the surface of a sphere or spheroid. A geographic coordinate system definition includes a datum (based on a spheroid), prime meridian, and angular unit of measure. The spheroid defines the size and shape of the earth model, while the datum connects the spheroid to the earth's surface. A GCS is based on a Spherical coordinate system. The term is popularly used in ArcGIS cluster of software.

What is a Projected Coordinate System (PCS)?

A reference system used to locate X, Y, and Z positions of a point, line, or area feature in two or three dimensions. A projected coordinate system is defined by a geographic coordinate system, a map projection, any parameters needed by the map projection, and a linear unit of measure. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the dimensions. PCS is based on a Cartesian coordinate system.

Why don't my datasets line up in my GIS platform (ArcMap, QGIS etc.)?

The major reason why datasets that do not overlay each other correctly when added to your map view is the presence of a discrepancy in the projection of the datasets. This may occur by one of the following factors:

1. The data frame’s coordinate system differs from the added layers
2. One or more of the layers have a different coordinate system
3. One or more layers have an incorrectly defined coordinate system

Ways to fix the above problem:

1. On-the-fly projection: Just use the Data Frame Properties dialog box to set or change the projection of the data frame to match that of the layers. When this is done, all layers with a correct coordinate system will be projected on the fly to the new coordinate system. On-the-fly projection also occurs when the first layer with a defined projection is added to the map view; by default, the data frame assumes the projection of that first layer. All other layers that are added will automatically conform to the data frame’s coordinate system. When this occurs, in most cases, layers stored in different but correct coordinate systems will be displayed as if they have the same coordinate systems.

2. Notwithstanding, there are times when on the fly projection may fail in making layers line up correctly. This can occur when the first layer added has an unknown projection or its projection is not defined. Fix this by restarting ArcMap and make sure to add a layer with a defined projection. The layers added subsequently will be automatically projected to the data frame’s coordinate system as long as they have defined coordinate systems. It is worthy to note that on the fly projection is only useful for display and query purposes.

3. Define projection: Find all the layer’s with an unknown or undefined coordinate system. Usually, ArcMap gives a warning whenever such layers are added. But in case you missed that, you can still check by looking at the layer properties.  After finding the “culprits”, use the define projection tool to set their coordinate systems.

4. Reproject: Projection discrepancies can also be fixed by totally changing a layer’s coordinate system into a new one so it conforms to the rest. To do this, one needs to use the project (data management)­ tool – for vector datasets or project raster – for raster datasets.

What is the difference between define projection and project tools in ArcGIS?

To define projection simply means to set the coordinate system (CS) of a layer whose CS is unknown, undefined or incorrect. This process does not significantly alter the layer: no changes in coordinates – changes in only the definition label; original data set is kept intact. Apply the define projection tool only when a layer’s CS is unknown, undefined or incorrect.

To project, on the other hand, refers to a change from one coordinate system to another. This process, unlike define projection, causes a significant transformation in the spatial reference of a layer: changes the coordinates as well as the definition label; creates a new data set. Use it only when changing a coordinate system permanently.

How do I choose a projection which is most appropriate for my project?

No map projection is perfect; each has some distortion. The type and degree of distortion vary with map projections. There are four recognized forms of distortions: Area, Distance, Direction, and Shape. These together form the pneumonic, it all ADDS up. In choosing a map projection, one can be guided by the following:

1. Which map property do you want to preserve?

If                     then choose a/an….. based projection

Area                Equal area

Distance          Equidistant

Direction         Conformal / Azimuthal

Shape              Conformal

2. What is the shape or orientation of the study area?

If                      then choose a/an….. based projection

North-South     Universal Transverse Mercator (UTM)

East-West         Conic Projections

3. What is popularly used (by convention)? You can find out by checking maps created by recognized agencies like the United States Geological Survey (USGS), Environmental Systems Research Institute (ESRI), US Census Bureau, the African Development Bank (ADB), major government bodies etc.

Where can I find information about layer projections?

Information about a dataset’s projection is usually contained in the associated metadata. Therefore, whenever you download or obtain a dataset, make sure to get the metadata as well. Also, when you generate your own datasets, make sure to create metadata to assist other users to understand and use the data correctly and effectively.

What is the difference between ArcView, Arc Editor, ArcInfo, etc? 

All of them are functional components of ESRI's suite of GIS software for Desktop called ArcGIS. The differences in names are representative of the varying license levels among them - ArcView, ArcEditor, or ArcInfo (now called Basic, Standard, and Advanced respectively). These license levels share the same core applications, user interface, and development environment (which includes ArcMap – for analyzing, displaying, and mapping data; ArcCatolog - for creating, exploring, and managing GIS datasets, and ArcToolbox – contains all the functional tools). Each license level provides additional GIS functionality as you move from Basic to Standard to Advanced.

Opening and Saving ArcGIS across versions

Each new version of ArcGIS introduces functionality and properties that aren't available in previous versions. As a result, once you open and save an existing map document (.mxd file) based on an earlier version (eg. ArcGIS 10.4) using a newer version (eg. ArcGIS 10.6), the map can no longer be opened with the earlier version of ArcGIS. The reason is that, your newly saved work will now reflect the new functionality added to the newer version of ArcGIS. Note that, map documents saved in older versions of ArcGIS can be opened in newer versions but not the other way round.

Notwithstanding, you can use the Save A Copy command to make a copy of a map document so you can open and work with it in previous versions of ArcGIS. With ArcGIS 10.6, you can save to ArcGIS 10.5.1 - 10.1, 9.3, 9.2, 9.0/9.1, or 8.3. Note that documents from ArcGIS 10.1 - 10.6 and related releases such as 10.3.1 are directly compatible with each other; ArcGIS 9.0 and 9.1 documents are also compatible with each other.

Saving to a previous version, however, may sometimes cost you something. It sometimes removes from the file any functionality that depends on the newer software. Therefore, some work may be lost if you save, for example, 10.6 to 9.3, 9.2, 9.0/9.1, or 8.3 and start working with the older copy again in 10.6, since the 10.6 functionality was stripped out in the Save A Copy process. Your original ArcGIS 10.6 file will still have the new functionality.

In addition, when you save a map document to a previous version of the software, only the .mxd file is saved; the data sources referenced in the .mxd file remain unchanged. See the following sections for more information about geodatabases, data sources, and saving to previous versions.

How to save a map to a previous version of ArcGIS

Steps:

1. While in the newer version (e.g. 10.6), click File > Save A Copy.
2. Navigate to the location where you want to save the map document.
3. Type a file name.
4. Click the Save as type drop-down arrow and click ArcMap 10.5 Document, ArcMap 10.4 Document etc.depending on which version you want to save.

Note: If you choose ArcMap Document (the option without a version number), the map will be saved in the current version of the software.

5. Click Save.

Note:

• The older version map document will be saved to disk, and your ArcGIS 10.6 document will remain open.
• If there are any layers in your current document that the previous version won't be able to draw, a dialog box will appear listing them. You can then decide whether to continue with saving the copy in ArcGIS 10.5 - 8.3 format.

Tip:

• The Save A Copy command is different from the Save As command. The Save As command allows you to save your document with a new name or file location. When you use the Save As command, the new Save As map project is loaded as the current document in the application.
• With the Save A Copy command, you are saving a copy of the document to disk, and the document is not reloaded in the application. In addition, the Save A Copy command can also be used to save a document so it can be opened in a previous version of ArcGIS.

Source: ESRI, 2012

I can’t see my toolbar or the Table of Contents. What should I do?

To enable or retrieve a toolbar, go to the top menu bar and right-click an empty space. A long list of toolbars will appear. Select the one you are looking for. Note that most of the basic commands such as Add data, zoom in, full extent, and clear selected features are found within the Standard and Tools toolbars.

Regarding the Table of Contents, go to the top menu bar then click on Windows à Table of Contents. You should now see your Table of Contents window.

I need my map as a picture (in .jpeg, .png etc format) so I can paste in my Word document or PowerPoint presentation. How can I do that?

First, create and design your map to your satisfaction. Next, click on the File menu à Export Map. Now click the “Save as type” drop-down arrow and select from the different image formats available e.g. JPEG, GIF, PNG, TIFF, EMF etc. Each format has varying suitability for specific purposes. A brief description of the common ones is given below. Check this link, export map for more information.

• JPEG (Joint Photographic Experts Group) - They are compressed image files that support 24-bit color. They are a good choice for Web uses because they provide control over output quality and size and can be more compact than many other file types.

• GIF (Graphic Interchange Format) - They are a standard raster format for use on the Web. They are a good choice for maps that contain a limited number of colors, but may not display raster data correctly due to the color limitation.

• PNG (Portable Network Graphics) – They are a raster format designed for use on the Web. It supports 24-bit color and is compressed too.

• TIFF (Tagged Image File Format) - These are the most versatile raster format. TIFFs can store pixel data at several bit depths and can be compressed with any of a selection of compression techniques. They are the best choice for importing into image editing applications across operating systems.

• EMF (Windows Enhanced Metafile) - These are also native Windows graphics files that can contain a mixture of vector and raster data. They are useful for embedding in Windows documents because they can be resized without loss of quality.

The GIS Services recommend JPEG, or PNG for figures to embed in Word document and EMF for figures in PowerPoint.  Next type a file name for the file (map) you are exporting and click save. Now, outside ArcMap, navigate to where you saved the map, select it, right click and copy. Go to your word document or PowerPoint presentation and paste. If necessary, resize your image. In case you found an error or you are not pleased with the map, you can go back into ArcMap to fix it by repeating the above steps.

I need / would like to create a map for my project. Where do I start?

First, you need to consider the following:

• What is the purpose of your map?
• What information do you want to communicate?
• What is your area of interest (study area)?
• Who is your target audience?
• Mode of presentation? Research Paper, Conference Presentation or Web?
• Do you have the relevant datasets available?

Next, you will need a GIS software to create the map. If you do not have one, you can visit the GIS lab to use our map software. These software are highly technical and require some prior training before using them. The GIS Services provides periodic workshops on selected topics how to use these software e.g. Basics of ArcGIS.  You can learn some great skills through these workshops. If due to lack of time you are unable to attend any of our workshops, you are more than welcome to visit the GIS Services for help with your GIS questions. The GIS Services Specialist is ever ready to assist you with your mapping or GIS needs. You can also schedule an appointment for one-on-one assistance.

For further help with using ArcGIS tools and functions, watch this video playlist on Youtube.

What is ArcGIS Online?

ArcGIS Online is a web-based (cloud-base) collaborative GIS operated by ESRI that allows patrons to use, create, and share maps, scenes, apps, layers, analytics, and data.

How much does it cost to use ArcGIS Online?

ArcGIS Online is based on an annual subscription that offers a set of plans from which users can choose. Each plan includes a number of named users and service credits. Service credits are used in exchange for premium hosted services such as storage of hosted web layers, performing analytics, and using demographic maps.

Can one access ArcGIS Online for free?

Yes. With a free public account, one can create, store, and manage maps, apps, and files, and share them with others. You also get access to content shared by Esri and GIS users around the world. Public accounts are for non-commercial use only.

What is an ArcGIS account and how is it different from an Esri Account?

An ArcGIS account is either a public or an organizational account that provides access to ArcGIS whereas an Esri Account provides access to Esri web resources such as My Esri, ESRI Virtual Campus and GeoNet. An Esri Account also serves as a public account that could access the public components of ArcGIS. An ArcGIS public account has automatic access to Esri web resources whereas an organizational account requires administrator permission to access Esri web resources by enabling Esri access for such account.

What are the differences between ArcGIS and QGIS?

How are the following terms different from each other: a GPS, GIS, GIScience and Geocomputation?

A GPS (Global Position System) is a network system comprised of radio-emitting and -receiving satellites used for determining positions on earth. Positions are determined through a process of trilateration. GPS was developed and is currently operated by the U.S. Department of Defense. The Russian counterpart of the United States Global Positioning System is the Global Navigation Satellite System (GLONASS).

GIS (Geographic Information System) is simply a computer based system for capturing, managing, analyzing and displaying location-linked data.

GIScience (Geographic Information Science) according to Mark (2003) is "the development and use of theories, methods, technology, and data for understanding geographic processes, relationships, and patterns."

Geocomputation is the application of advance computer technology in solving spatial problems. It often involves spatial analysis, spatial modeling, simulations and geovisualization.

What is spatial statistics (also called geostatistics)

The field of study concerning statistical methods that use space and spatial relationships (such as distance, area, volume, length, height, orientation, centrality and/or other spatial characteristics of data) directly in their mathematical computations. Spatial statistics are used for a variety of different types of analyses, including pattern analysis, shape analysis, surface modeling and surface prediction, spatial regression, statistical comparisons of spatial datasets, statistical modeling and prediction of spatial interaction, and more. The many types of spatial statistics include descriptive, inferential, exploratory, geostatistical, and econometric statistics.

What is LiDAR data?

LiDAR is an acronym for Light Detection And Ranging. It is one of the most reliable remote-sensing technologies employed in producing surface models by using laser pulses emitted from an aircraft like a helicopter or plane. The captured data are often used to produce highly accurate digital terrain models.

USGS - Spectral Characteristics Viewer

Wondering what spectral bands are best to use for your study? Check out the Landsat Spectral Characteristics Viewer. You can select different sensors and compare the bands to the spectral characteristics for different minerals, vegetation, or water.