GIS Fundamentals: Introduction to GIS
Lesson 3, Digitizing
Lesson 3: Digitizing in ArcMap
What You’ll Learn: In this Lab you’ll be introduced to basic digitizing techniques using
ArcMap. You should read Chapter 4 in the GIS Fundamentals textbook before starting
this lab, as the chapter covers the basics of data entry, properties of hardcopy maps,
and digitizing mechanics.
Data are located in the \L3 subdirectory, including Boat_Docks.shp, Houses.shp, and
Islands_in_River.shp, files with errors that you’ll fix, and Lab3_image.img, an image file
that you’ll use as a data source. All data are in UTM Zone 15, NAD83 coordinates.
Manual Digitizing
Digitizing is the process of interpreting and converting paper map or image data to
vector digital data. In manual digitizing you trace the lines or points from the source
media. You control a cursor, usually with a mouse or digitizing puck, and sample
vertices to define the point, line, or polygonal features you wish to capture. The source
media may be hardcopy, e.g., maps taped to a digitizing table, or softcopy, e.g., a digital
image or scanned map. ESRI software allows us to digitize using either hardcopy or
softcopy sources.
On Screen Digitizing in ArcMap
Our practice exercise will involve digitizing a set of features from a scanned photo.
Digitizing directly on screen is sometimes called “heads-up” or softcopy digitizing.
Flatbed scanners and digital cameras are common, so softcopy digitizing is a standard
procedure. Scanned photos have some geometric distortion that depends primarily on
how the photo was taken (flying height, terrain, camera tilt, and other factors). GIS data
are commonly entered from scanned photos because we can easily adjust the display
scale, zooming in or out as needed. This often reduces both interpretation and digitizing
errors.
Setting File Properties
Digitizing changes the contents of files, so you must have permissions to modify old
files, or to create new files in a workspace. Data on the hard drive may be ‘write
protected”, so that they may not be edited. The computer administrator may have
denied access to certain data, or only allow read access, to protect these data from
inadvertent modification.
This protection sometimes becomes a problem if you wish to edit data, because you
need to write data when editing. You might think you can avoid this
problem by copying the data to a portable drive, but often the
protection properties are transferred with the data. Data on a USB drive typically are not
write protected, and if they are, by default you have permissions to change this. To see
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Lesson 3, Digitizing
the permissions, open Windows Explorer and navigate to the hard disk where the data
are stored (specified by a drive letter).
Notice there is a number of files listed.
With most versions of the MS Windows
operating system, you may observe the
properties by:



Single left-clicking a file
Then, right click over the
selected file
View the popup window, and
select Properties,
near the bottom
This will display various properties for the file (see
at right). Note that near the bottom-left of the menu
there is a check box under read only.
 You should make sure (by left clicking in
the box) that this checkbox is blank.
 With the checkbox blank, left click on the
Apply button to the lower left of the window. The
file will now be write-enabled
You may also adjust folder properties, and apply
these properties to all files in the folder.
To select a folder single left clicking on the folder.
Then hold the cursor over the folder, right click, and proceed as above.
Remember to set the path names as relative. If you are going to be working across
multiple computers, for example, doing some work at the office and other work at home,
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Lesson 3, Digitizing
then you should also be careful to have the data and project saved in or near the same
location, and to set the project to have relative paths. See Lab 1 for instructions on
setting paths for a project, for details on how to set a relative path.
Also remember that you should set your default home workspace/folder, and create a
default GeoDatabase within that home. In summary, before you get started you should:
Use Windows Explorer to create a directory (e.g., \Lesson3), preferably on your portable
USB drive. As shown in an earlier lesson, do the following:

Start ArcGIS and create an empty/new project (remember, if necessary select a
new, blank map from “My Templates”)

Save the project to the just created directory (left click on File in the upper left of
the main ArcGIS screen, then left click on Save, and navigate to the directory you
created above, e.g., \Lesson3. This makes it the home directory for this project)

Set your path names relative to that directory (remember, File, then Map
Document Properties, then check the relative path box at the bottom)

Create a new GeoDatabase in the home directory (ArcCatalog near the upper
left of the main ArcGIS window, then left click on the Home - folder, then New,
then File or Personal GeoDatabase, then type in a name when prompted)

Left click on this new GeoDatabase and make it the Default GeoDatabase
Digitizing
Start ArcMap, and
create a new, blank
document.
Select the add Add
Data button;
navigate to the \L3
folder, and add the
image named
lab3_image.img to
display something
similar to the figure
right.
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Lesson 3, Digitizing
The image is a scanned True Color photograph of the type often used for land
cover and vegetation mapping. Notice how the coordinates change in the lower
right corner of the ArcGIS window whenever you move the mouse. These are
the coordinates for the view, and they have been established from the image
data. Also note and practice the zoom (magnifying glass) and pan utilities
(hand), activated by the icons on the toolbar, shown with a vertical orientation, at
left. Note that the toolbar can also be horizontal, as shown in the figure below.
The toolbars can be either vertical, or horizontal, because they are dockable. This
means you can grab them with a left click of the mouse, and drag them to another
position around the frame. You can also make them free floating, by dragging them out
into some part of the window, and then letting go with the left mouse button.
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Lesson 3, Digitizing
Create a New Shapefile or GeoDatabase/Feature
Dataset/Feature Class Using ArcCatalog
Open ArcCatalog by clicking on the icon in the main
ArcMap toolbar
This should open a window with two panes, a navigation
panel on the left, and a file display on the right.
Use the left pane to navigate to the L3 subdirectory.
Left click on File at the top
of the ArcCatalog window,
and then in the dropdown
window, select New, then
Shapefile (see the figure
at right)
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This should open a “Create
New Shapefile” window.
You have entry boxes for a
Name, and a Feature Type
(see the figure right).
The default feature type will
initially come up as point, but
you could also select line or
polygon as the default types,
using the selection triangle to
the right of the Feature Type
block.
Enter something descriptive
for a Name, e.g., Ponds. If
you leave the default
“New_Shapefile” name you
may have difficulty keeping
track of your files.
Specify a Feature Type of
point
Left click on Edit to set
the coordinate system.
The resultant set of windows
will be much like the
projection windows already covered in Lab 2.
Specify Select > Projected Coordinate System > UTM > NAD83> Zone 15N system.
You may also specify a coordinate system from an existing layer, via Edit then Import,
and navigating to the Lab3_image.img data set, or any existing datasets with the
desired coordinate system. See Video Create Shapefile for a detailed demonstration.
Create a new shapefile of type polyline, to hold the roads you will digitize, and a new
polygon shapefile into which you will digitize fields. Name these appropriately, and
assign the UTM NAD83 Zone 15N coordinate system.
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Lesson 3, Digitizing
*************** Begin Optional Section ***************************************
Instead of using Shapefiles, you may also wish to create a GeoDatabase, the more
complex, but powerful data forms that were introduced at the end of Exercise 1.
Video: Create GeoDatabase
Again, in ArcCatalog, left click on the target directory (Lab3 or L3 here, or what you
named your directory on your working drive), but this time select New -> Personal
GeoDatabase. This will create a new GeoDatabase in the directory tree of the
ArcCatalog window, and the name will be open for editing. Rename it, here I named it
ddigitlayers. Note the .mdb extension is automatically appended for you.
Right click on the
ddigitlayers.mdb in the
ArcCatalog window, and then
select New, Feature Dataset.
This will generate a set of
windows, first to specify the
Name (use “Lab3 Creations” or
something similar, then right
click on Next)
The coordinate system (use
UTM NAD83, Zone 15 N, then
Next)
Then the Vertical Coordinate System
(choose None, then Next)
Then for Tolerances (take the default,
then select Finish). If you’d like to
know what tolerances are, click on the
“About Setting Tolerances” button on
final window.
this
Now we need to create the point, line,
and polygon layers we’ll digitize into within this Feature Dataset.
Left-click on the “Lab3Creations” feature dataset you just created, displayed in the
ArcCatalog window, then select New, and Feature Class.
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This will open a window
which allows you to name
the new feature class, and
define the Type. Here we’ll
create a point layer named
pondsGDB, here named
distinctly from the shapefile
version of the ponds we’ll
create, but it could have the
same name as a shapefile
in the same directory,
because this feature class
is stored within a feature
dataset.
Click Next, and it displays
the fields and field
properties for the data table
for the layer. You would type them in a row and select the data type here. For now,
accept the defaults, and select Finish.
Note you didn’t have to specify the coordinate system for the feature class layer.
Because it is contained within the feature dataset, it has the coordinate system you
specified for that feature dataset.
Now create two more feature classes within this
Lab3Creations feature dataset, one a line layer named
“roadsGDB,” and a polygon layer named “FieldsGDB.”
When you are done, the feature dataset should look
something like the figure on the right in ArcCatalog:
Note that the icon shows the type for each feature class,
with three points next to pondsGDB, a line icon next to
roadsGDB, and polygons next to FieldsGDB.
************ END OPTIONAL SECTION ********************
Digitizing Features
Close the ArcCatalog window (click the X in the upper right corner), and add the
lab3_image.img to your data frame. Your goal is to digitize points (ponds), lines (major
roads), and polygons (selected fields) from this image. See the map at the end of this
document labeled “Digitized and Corrected features in Stillwater Township, 2006” for
the minimum number of roads, ponds and field you need to digitize; you can do more if
you wish. You’ll need to zoom in and pan around to discover all the features on the
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Lesson 3, Digitizing
map. If you color image disappears, select the lab 3_image.img layer, right click and
zoom to layer.
These steps are demonstrated in the Video: Digitize Points.
Add the Ponds, Fields & Roads layers to your ArcMap,
either the shapefile, or the GeoDatabase version. Make
sure the new, empty point, line, and polygon layers appear
in your table of contents.
Left click on Editor, Start Editing
A window pops up that shows the available layers in a top
panel, and the source directories/GeoDatabases and types
in a bottom panel. You select a target layer by left-clicking
on it; it should then be hi-lighted in blue. You should click on
the “About Editing and Workspaces” button in the lower left,
and read the description. (Note, this Window may not appear if all the
layers are in the same directory)
Select “ponds” (shapefile version) or “pondsGDB”
(GeoDatabase version) as your target layer to digitize, and
then click O.K.
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A new “Create Features” window will
display (see at right), either freefloating, or docked on the right edge
of your workspace.
Left-click on the ponds layer as the
target for edits, a blue box will appear
around the name.
Note that the “Construction Tools” in
the bottom panel of the window
change as you select different types
of features. Make sure you have
ponds selected, and it is a point
feature, with Construction Tools as
shown at right.
Find the Editor Toolbar, usually a horizontal series of icons near the upper-left side of
the ArcGIS window (see below).
The bar may be vertical, it is detachable, and may be set up as free floating, or along
the left or right edge.
The icons allow you to add or modify features. Note that not all icons work for all
features, some are just for points, some just for lines, some only for polygons.
Note that you may select a tool by clicking on it in the toolbar, or that some tools are
selected by clicking on one of the choices in the Construction Tools menu. Alternate
(with successive left clicks) on the Point and Point at end of line icons in the
Construction Tool menu, and note how the active tool in the toolbar, denoted by a box
around the tool, changes position.
Make sure the add point tool is activated.
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Use the pan and zoom tools to zoom to inspect the image. Note that there are several
ponds, dark blobs in the fields, often surrounded by a light- and/or brownish-colored
ring. Your task is to digitize at least 20 ponds; we’ll get to that in a minute, but first a
few words about scale.
Scale is important when digitizing. Too small a scale magnifies errors, e.g., 1:50,000.
Too large a scale is inefficient, e.g., digitizing at 1:100 you will be very accurate but will
require frequent panning. The best scale depends in part on the level of accuracy you
need, and how the ponds or other objects appear on the images.
Right click on the Lab3_image in the TOC and select Zoom to Raster Resolution.
Notice you now zoomed to something like 1:1,200. This is probably larger than needed
for this Lab. You can directly enter a scale; here use 1:4,000, a good compromise (as
shown in a previous Lab, simple type 4000, without the comma or the 1 and colon, in
the data view scale box). (Video, Set Scale to Digitize)
Pan until there are several ponds visible in the image (remember keep the zoom near
1:4,000.
You use the point tool
to add points. If it isn’t activated, click on the pondsGDB
(or whatever you named it) in the create features window, and if need be the
Point icon
in the Construction Tools window.
Note the cursor now changes when you move it over the image, to a hollow arrow with a
shadowy point at the end. This indicates your left-clicks will add data.
To digitize points place the cursor over a pond, and single left-click. Each click creates a
feature.
Navigate and digitize at least 20 ponds as new points. Place the point in the middle of
the pond by positioning the cursor and left-clicking.
If the default point symbols are difficult to see, you can change the symbol color, size, or
type by double clicking on the legend entry for the layer in the table of contents, as you
would for any theme.
Note that when you select other functions during digitizing, e.g., changing the color or
size of a symbol, you will often have to re-activate the point data layer (here pointGDB),
and click on the kind of feature in the Constructions Tools you’d like to add, and then
continue digitizing.
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You should left click on Editor > Save Edits every few features, and when you are
done digitizing ponds. ArcGIS can crash, and you’ll lose any unsaved work.
Note the caret tool on the Edit Toolbar
. The caret is a selection tool. You use the
caret to select and edit existing features or parts of features.
When you activate the caret, the cursor appears as a caret, indicating that clicks will
select features.
You typically select features to either delete, or modify them. Hold the cursor over a
feature while the caret is active, and left click on the feature. Note that it changes color,
typically cyan, the default “selected feature” color.
A left click and hold allows you to drag a feature, try it with a few points.
A left click to select a feature, then a right click over the selected feature will display
various options, most important among them an option to Delete the feature.
Use the caret to move and delete point features while digitizing.
To de-select feature(s), left click on ArcMap main Selection menu and left click on
Clear Selected Features. The color should now match the rest of the features.
Finish digitizing all your ponds, save your edits, and stop editing (click on the dropdown
control in the Editor Toolbar).
We’ll now digitize line features, into the roads data layer.
Video: Snapping and Digitizing Lines.
Set Snapping
First we must set the snapping environment. As with many actions in ArcGIS, this is a
bit complicated. You can use the new snapping tool, or the classic snapping tool.
To use the new snapping tool,
first:
Left click on Customize along the
top of the main ArcGIS window,
then click on Toolbars. This will
display a dropdown list of choices.
Find Snapping, and left click on it.
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This should show a checkmark to the left of the name.
This display the snapping toolbar, also a floating toolbar, so it may appear anywhere
and you may click and drag to reposition it
Now click on the snapping dropdown pointer, and
then select Options:
This will display a window that allows you to set
options, most importantly the snapping tolerance.
Unfortunately, we can only set it in pixels, the
default units it chooses because we have an image
loaded.
We can calculate the tolerance distance we want,
but we have to convert the distance we want (e.g.,
4 meters) to the proper number of pixels. If you
don’t know the units for the image, we can look
them up (remember, right click on the image in the TOC, then Properties, then Source).
Verify that the image has about a 60 cm resolution, so for 4 meters we’d like to specify 7
pixels.
Many folks choose “Classic Snapping,” for among other reasons, because it allows us
to specify the units in real-measurements, e.g., meters, and provides a more obvious
display of the features participating in the snapping.
To activate Classic Snapping, leftclick on Editor, then select Options
from the bottom of the dropdown
menu, then place a check in the box
next to “Use classic snapping” (see
the figure, at right).
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To set the snap tolerance and units,
left click on Editor, Snapping,
Options (see right)
This will display a window labeled
“Classic Snapping Options,” shown
below, that allows you to set the
snapping tolerance in map units or
pixels.
We recommend you use one, or the
other, so if you choose to use classic
snapping, remember to turn off the Snapping Toolbar (via Customize, Toolbars, and
then click again on Snapping to toggle off the check mark).
Set snapping for something like 4 to 6 meters.
One final step to set up snapping, we need say what layers will be snapped.
Left click again on Editor, Snapping, as in the figure at above right, but this time select
Snapping Window. This will display the layers, and what features will participate in the
Snapping Environment. Left click to check the Vertex, Edge, and End boxes for the
roads layer, so that these elements of a feature will snap:
Now specify the roads layer as our digitizing target, select the roads layer in the Create
Features window and then select Line in the Construction Tools window.
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This should activate the line tool in the Editor Toolbar:
You’ll digitize the set of roads shown in the map at the end of these instructions. Set
your scale to something like 1:4000 to 1:8000.
Move the cursor to the start of the road you wish to digitize, and left click. Move along
the center of the road, left-clicking when you need a vertex. Note that this shows both
the vertices (knots on the path) and the “thread” connecting these knots. This is your
digitized line.
Double click to end a line. If the entire road segment you wish to digitize is not in the
view, navigate around the image using the pan and zoom tool. Note that you first pan
or zoom, and then you must click again on the target feature (roads, here) in the Create
Features window to return to the digitizing cursor. If you are digitizing and the road
continues beyond the edge of the view, the feature is still active, and you can add to it,
after you click on the layer again in the Create Features window.
You may need to refresh after a pan and zoom, using the refresh button next to the data
and layout view buttons:
Fixing Mistakes
What if you make a mistake while digitizing?
Placing the sketch cursor
over the last point
digitized, and right clicking reveals a dropdown
menu, as shown at right. You may then select
Delete Vertex, but only to remove the last vertex
digitized.
If there is a sequence of bad vertices, you can
move backward, positioning the cursor and right
clicking over each one in turn.
If you wish to remove an entire line, you may
-terminate the line (double click with the sketch
tool to end digitizing), then
-single left click on the line with the caret tool,
then
- right click and select Delete from the
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Lesson 3, Digitizing
dropdown menu (not shown).
You may select several lines by holding down the shift key, and left clicking on each of
them to select, then single right-click and Delete.
If you want to reshape a line, either to move or add vertices, you can double left click on
the line, which will display a segment editor toolbar:
Double left-clicking on a line segment reveals the
vertices. If you select the leftmost caret, and hold it over a vertex, you can left-click and
drag to move the vertex. The caret-plus and caret-minus icons allow you to add or
delete a vertex in a selected segment.
Digitize and fix mistakes until all indicated roads are digitized. Remember to set and use
your snapping tolerance so you don’t have under and overshoots. Save frequently
using Editor > Save Edits, and stop editing when you’ve digitized the roads shown in
the example map below. You can digitize more than the example map roads, but you
don’t have to.
Change your Create Features layer to the empty polygon data set you created, and set
the Construction Tool to Polygon.
Digitize the fields shown in the example map. Video: Digitize Polygons
Move the cursor to a border of the
field you wish to digitize and left-click
the mouse. This starts a new
polygon.
Each time you left-click, you will be
placing a vertex for a polygon, until
you double click, which closes the
polygon.
Move the cursor to the next field, and
repeat the digitizing process.
Continue digitizing features, at least
as many as are shown in the sample
map at right.
Save frequently using the Save Edits
option of the Editor Toolbar and Stop
Edits option when you are done.
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Editing Shapefiles
Add the following shapefiles to your data view:
Boat_Docks.shp
Houses.shp
Islands_in_River.shp
Video: Edit Shapefiles
Specify a display scale of 1:1500, because we’ll need to view these data with a high
magnification to edit them.
These layers have errors. The houses are not in the correct places, the Islands include
water, and the boat docks are unconnected or in the river. You will fix all three, using
the tools you learned above.
Set the Create Features layer to “Houses”, and use the caret to reposition the house
features to the locations of houses in the images. You may need to add and/or delete
house features, using the tools learned above.
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Load and display the Boat_Docks. Set the snapping tolerance, to something like 2 to 3
meters.
Start editing, and fix the two errors:
-
the unconnected docks in the northwest corner (upper right in the image)
trim the too long dock at the southern end (bottom of image)
Save your edits and stop editing.
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Finally, fix the Islands_in_River shapefile. Load it, perhaps change to a hollow
symbology, and set the snapping tolerances.
The four errors to fix, starting from the upper left corner are:
-
delete the spurious island
reshape the island to match the tree/water boundary
remove the overlap, and create one island from the two pieces
split the polygon, removing the water from the middle and creating two distinct
polygons.
You may create new features or add vertices and reshape features as described above,
as well as splitting, reshaping, and other tasks that are accessed through the Editor
Toolbar.
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To split a polygon into two parts, first left click on the island with the caret
it.
to select
Activate the polygon split tool
Place the cursor to one side of the polygon at the point you want to cut, left click, and
add vertices to define a path through the polygon, ending the path on the other side of
the polygon, clicking to define the path as you go.
Double click to end the cut, splitting the polygon into two polygons, which you may then
select, delete or modify.
Fix all the errors in the layer, saving and stopping editing when you are done.
Results
Below is the “approximate” Key for your digitizing. Digitize the Ponds, Road and Fields
and correct the Houses, Docks and Islands, and produce a map that looks
approximately like
the image below.
Remember to
include a North
arrow, legend, title,
your name, and
scale.
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