IMPROVING GIS SPATIAL ACCURACY WITHTHE GEOGRAPHIC COORDINATE DATABASE

Adjusting GIS Data to the GCDB

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Montana’s Cadastral Data Are Based On & Connected To The GCDB

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BECAUSE THE GCDB WAS NOT UNIVERSALLY AVAILABLE DURING EARLY MAPPING EFFORTS,

MANY BOUNDARY GIS LAYERS WERE CONNECTED TO OTHER GIS REPRESENTATIONS OF THE PLSS.

IN ORDER TO CONFORM TO TODAY’S CADASTRAL DATA, AND THE OFFICIAL REPRESENTATION OF THE

PLSS (THE GCDB), THOSE DATA MUST NOW BE CONNECTED TO THE GCDB.

THIS WORKSHOP WALKS THRU THE STEPS TO CONNECT A GIS LAYER TO THE GCDB.

All Boundary Data Should Be Connected to the GCDB

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THE OBJECTIVE OF THIS WORKSHOP IS TO INFORM ON HOW TO CONNECT GIS BOUNDARY DATA BOUNDARY DATA TO THE

GCDB.

Objective4

Workshop Steps

Lesson 1: Gather & Organize the resources.Lesson 2: Prepare the data for the

adjustment.Lesson 3: Perform the adjustment.Lesson 4: Analyze the results.Lesson 5: Follow up editing.

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Graphic of the Basic Adjustment Process

GATHERING & ORGANIZING THE RESOURCES

Lesson 17

Montana’s GIS/GCDB Web Page8

http://giscoordination.mt.gov/cadastral/AdjustingDatatoGCDB.asp

•White Paper•PowerPoint Overview•PowerPoint Tutorial•Adjustment Script•Links to GCDB Information•Links to GCDB Data.

GIS Data to AdjustGIS Data to Adjust Reference & Source DataReference & Source Data

Boundary GIS layer GCDB Points (New Control)

Vertices from the Boundary GIS layer (Old Control)

Cadastral Adjustment Script

Adjustment Data & Resources9

Tutorial Data

The example GIS data set is Pondera County’s Fire Districts. This dataset is not connected to the GCDB as shown in the accompanying illustration.

The goal of this tutorial is to connect this GIS dataset to the GCDB.

Pondera County Fire Districts

GCDB points

GIS Layer vertices not coincident with GCDB

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Gathering the GIS data sets

GIS Data to AdjustGIS Data to Adjust

1.1.Create a project workspace;Create a project workspace;

2.2.Copy the boundary data to workspace folder;Copy the boundary data to workspace folder;

GIS Data to AdjustGIS Data to Adjust

1.1.Create a project workspace;Create a project workspace;

2.2.Copy the boundary data to workspace folder;Copy the boundary data to workspace folder;

GCDB Reference DataGCDB Reference Dataftp://ftp.gis.mt.gov/cadastralframework/GCDB

Copy the GCDB file to the workspace folderCopy the GCDB file to the workspace folder

OR …OR …

GCDB Reference DataGCDB Reference Dataftp://ftp.gis.mt.gov/cadastralframework/GCDB

Copy the GCDB file to the workspace folderCopy the GCDB file to the workspace folder

OR …OR …

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Alternatively Connect to the Cadastral Map Service

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Connect to the GCDB map service via ArcCatalog:Connect to the GCDB map service via ArcCatalog:

gis.mt.govgis.mt.gov

Load the source data into ArcMap

GIS Data to AdjustGIS Data to Adjust

1.1.Open ArcMap;Open ArcMap;

2.2.Add the boundary data to ArcMap;Add the boundary data to ArcMap;

GIS Data to AdjustGIS Data to Adjust

1.1.Open ArcMap;Open ArcMap;

2.2.Add the boundary data to ArcMap;Add the boundary data to ArcMap;

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Load and prepare the reference data

GCDB Reference DataGCDB Reference Data1.1.Add the GCDB points to ArcMap;Add the GCDB points to ArcMap;2.2.Select the GCDB points that are within ½ mile of Select the GCDB points that are within ½ mile of

the boundary data;the boundary data;3.3.Save the selected GCDB points as a shapefile to Save the selected GCDB points as a shapefile to

the workspace folder.the workspace folder.

GCDB Reference DataGCDB Reference Data1.1.Add the GCDB points to ArcMap;Add the GCDB points to ArcMap;2.2.Select the GCDB points that are within ½ mile of Select the GCDB points that are within ½ mile of

the boundary data;the boundary data;3.3.Save the selected GCDB points as a shapefile to Save the selected GCDB points as a shapefile to

the workspace folder.the workspace folder.

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Add GCDB Points15

Select the GCDB

points that are within ½ mile of

the boundary data set.

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Export the selected GCDB points to a shapefile

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Make sure that all your data sets have the same

spatial reference

Make sure that all your data sets have the same

spatial reference

Understanding your data18

PLSS vs. Non-PLSS Boundaries19

Review your data quality

1. Review the spatial accuracy of the GCDB.2. Review the quality of alignment of the Old

Control to the New Control.

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About GCDB Accuracy

Each GCDB point represents the location of a township, or section, or ¼ section corner . Every GCDB point has an associated X coordinate accuracy estimate, and a Y coordinate accuracy estimate.

The GCDB will serve as the reference frame for the boundary data. Therefore the boundary data will be no more accurate than the GCDB.

Note: Other GCDB points may exist in some areas that represent locations in original PLSS surveys such as meander points along riparian areas, or mineral survey corners, and others.

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GCDB Accuracy Fields22

How to review the spatial accuracy of the GCDB.

1. Right click on the GCDB layer and open its table;

2. Right click on the ERRORN or ERRORE fields and select STATISTICS from the pop-up menu.

Your results should look similar to this:

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The error values are subjective estimates.

This is the range of errors of the coordinates in this GCDB data set.

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Another way to look at the magnitude and distribution of errors is to map them.

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PREPARING THE DATA FOR THE ADJUSTMENT.

Lesson 226

Control Point Files27

GCDB adjustments use 2 control point files.

1.The New Control is the GCDB (and sometimes additional points).

2.The Old Control comes from either an older GCDB (for data already connected to the GCDB), or from the vertices of the layer to adjust.

Two steps for creating the Old Control28

1. Create the points (from the polygon vertices).

2. Assign GCDB IDs to those points.

Additional steps and processing may include:•Weeding out inappropriate or unnecessary points;•Creating non-PLSS control points.

Points From Vertices

Points from Polygon vertices:

Generate points at the vertices of the polygon layer (Pondera Fire Districts).

The points will be separate shapefile of points that will be the basis for the oringination vectors in the adjustment

Call the shapefile OldControl_NoIDSOldControl_NoIDS

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To create a control file from the GIS features…

ArcINFO: ToolboxToolbox: Data Management Data Management FeaturesFeatures Feature Vertices Feature Vertices

to Pointsto Points– creates a point layer from the polygon, or polyline vertices.

ArcEditor or ArcView Use a third party utility such as ETGeoWizard or XtoolsPro to

create a point layer from the polygon, or polyline vertices.

The point layer will be used as the “OLD” control, so name it OldControl_NoIDs OldControl_NoIDs (we will add the IDs in a later step)

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ArcInfo31

Feature Vertices to Points

Save to a file in your workspace.Name the file OldControl_NoIDs OldControl_NoIDs

Alternatively:ET GeoWizard Polygon to Point32

OldControl vs New Control

The Cadastral Adjustment will use the coordinates of the OldControl points and the NewControl points to generate the adjustment vectors.

The basis for these vectors is the GCDB ID.

Assign the GCDB ID to each point in the OldControl_NoIDs

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GCDB point

Vertex point

Spatial Join

The Spatial Join will assign to each point in the OldControl_NoIDs, the attributes of the GCDB (NewControl) points nearest to it.

Join the NewControl to the OldControl_NoIDs

New ControlNew ControlGCDB points (red squares)

OldControl_NoIDsOldControl_NoIDsVertex points (green circles)

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Spatial Join1. Right click on the

OldControl_NoIDs in the TOC

2. Joins & Relates

3. Join

4. Select :

5. Join data from another layer based on spatial location

6. Select the NewControl as the layer to join to OldControl_NoIDs

Save the new file as

OldControlOldControl

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Myworkspace\OldControl

Results of the spatial join

GCDB IDs are now assigned to each point in the OldControl., along with the distance* to the nearest GCDB point (shown here in parentheses).

However, there may be some duplication, as shown here (and on the next slide).

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* Note that the distance units returned by this operation are the units set in your data frame’s spatial reference (probably meters).

Dealing with duplicates

Here 3 points in the OldControl have the same GCDB ID, however, only one is appropriate.

Discuss some ways to deal with this.

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The distances to the nearest GCDB point are shown in parentheses.

Example solution – Generalize Polygon

One way to deal with the duplicate IDs in the OldControl, is to clean up the polygon layer first, in order to eliminate or reduce the number of vertices.

This is a good solution in this example because the extra vertices should not be there.

Here is the generalized polygon showing the reduced number of vertices.

Instead of 3 vertices (green) in the original polygon, there is now only oneVertex (yellow) after generalizing the polygon.

ResultantSingle vertex(yellow)

Originally there were 3 vertices(green)

Nearest gcdb point

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Caution! Double Corners39

There can be double corners – especially along township lines. Make sure that you snap to the correct corner for each section!

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Other cleanup and weeding examples

Thin your OldControl File

1. Remove all points that are so far from the nearest GCDB point that they probably are NOT the same point. (use the distance field to identify them, e.g. distance > 60)

2. Remove all the meander points (“PointLab” like ’80%’)

3. Remove other points that are probably not PLSS points.

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Meanders vs vertices•This GIS boundary follows the river in parts.

Here we see the difference between the boundary vertices(green circles) versus the GCDB meander points (red squares).

•We do NOT want to use the meanders as control points, so we shall remove them (based on the IDs).

River Boundary Vertices vs. Meanders

42Thin your OldControl File

Remove meander points:

Meander Points

Meander corners are part of the original PLSS surveys along water ways. These points are represented in the GCDB with an 800 number series in the IDs.

Meanders typically do NOT represent water boundaries, and should be removed from the control point files.

Meanders along waterways are not boundaries.

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Other points to remove based on GCDB Code(PointLab)

GCDB ID Code Represents

000000 - 000009 Reserved for software codes

095000 - 704999 Rectangular Surveys

705000 - 799999 Boundaries with Mileposts

800000 - 836999 Riparian Boundaries

837000 - 899999 Tracts

900000 - 999989 Mineral Surveys, Homestead Entries, etc

999990 - 999999 Reserved for software codes

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Weed by distanceWeed by distance Remove meanders and other non essential pointsRemove meanders and other non essential points

How to Weed the OldControl Point File by selecting and deleting records.

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Notes

1. If you have points in the OldControl that do not have a GCDB ID, those points will not get an assigned adjustment vector. So, the polygons at that location will move an amount that is proportional to its distance to the nearest adjustment vectors.

2. If you have some boundaries that you want to hold fixed in place (e.g. river ways or road ways), then give them dummy IDs and COPY them to the NewControl file. This gives those points (and the polygon edges that follow them) ZERO adjustment vectors.

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Areas that should NOT move can be fixed in place

FIX in Place

1. Select the OldControl points on boundaries that are NOT PLSS, and give them unique, non-GCDB IDS.

2. Then, copy them into the NewControl file.

Some boundaries should remain where they are.

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After the cleanup, the files are now ready to adjust

The OldControl Is Now Ready for the Adjustment

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PERFORM THE ADJUSTMENT.

Lesson 351

Free(!) Script52

ftp://ftp.gis.mt.gov/cadastralframework/GCDB/AdjustmentScript.zip

http://giscoordination.mt.gov/cadastral/AdjustingDatatoGCDB.asp

Load the Cadastral Adjustment Script

1. Tools Macros Visual Basic Editor(or: ALT + F11)

2. In the Visual Basic Editor: (highlight Project)

1. File Import File RubberSheetLayers.txt2. Expand Project Modules Module1 [double click

on this]

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TIP:The cadastral adjustment script is a TXT file, so you may need to show ALL FILES in the file browser.

The script

Parameters to Modify

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Obtain & Modify the Parameters in theCadastral Adjustment ScriptCadastral Adjustment Script

You should check and/or modify the following:Your dataset names: OldControlOldControl, NewControlNewControlYour field names: POINTIDPOINTID (all caps, no

underscore)

Your radial search parameter (note that the unitsunits are the same as the data frame’s units).

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Application Requirements56

All datasets must have the same spatial reference/units, etc.

Editing must be active on the data workspace.

There must be two control point files: Old Control New Control

Control point files must both have a POINTID field with matching strings.

THE DATA SETS HAVE DIFFERENT SPATIAL REFERENCES.

THE NEWCONTROL (GCDB) FILE HAS RECORDS THAT ARE MISSING A POINT ID.

FIELD NAMES IN NEWCONTROL AND OLDCONTROL DO NOT MATCH EACH OTHER.

FIELD NAMES IN NEWCONTROL AND/OR OLDCONTROL DO NOT MATCH THE PARAMETER IN THE SCRIPT.

FILE NAMES FOR NEWCONTROL AND/OR OLDCONTROL DO NOT MATCH THE PARAMETERS IN THE SCRIPT.

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Some things that could choke the script…

Run the Adjustment

1) After setting your parameters to the appropriate values,

2) Start an EditingEditing session in ArcMap

3) hit the RUN button.

BACK UP YOUR DATA!BACK UP YOUR DATA!Be sure that you have a copy of your original polygon data, in case something goes wrong.

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Results!59

Original location

Adjusted location

ANALYZING THE RESULTS.

Lesson 460

Polygon movement to the GCDB

The vertex moved 5.2 feet here.

FOLLOW UP EDITING

Lesson 562

Transition Areas63

TRANSITION AREA BETWEEN PLSS AND NON_PLSS BOUNDARIES:Some additional editing might be necessary after the adjustment when non-PLSS features are boundaries because the linkages from the PLSS to the non-PLSS may be modified by the adjustment. Careful inspection of the adjusted data is very important for data integrity. For example, where a boundary follows a section line to a river as shown below.

Example Transition Area64

Adjusted boundary (green) snapped to GCDB point

Original boundary (orange)

Transition area

Note: New vertex introduced by adjustment

GCDB section line (red)

Detail of Example Transition Area65

OrangeOrange line is before the adjustment, greengreen line is after the adjustment.

PLSSLine

RiverLine

Transition area - requires review and editing.

Transition area - requires review and editing.

For more information contact

Montana Base Map Services Center: Stu Kirkpatrick – State GIS Coordinator

406-443-9013 skirkpatrick@mt.gov Keith Blount – Geographer

Montana Cadastral Mapping Project DOA\ITSD\GIS Bureau 406-443-9891 kblount@mt.gov

DJ & A, P.C. Rj Zimmer, PLS

406-443-9062 RjZimmer@djanda.com

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THE END

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